[paw broon]

Captain Triumph. Just to start you off.
Thompson and Thomson?
(didn't Jije' have a similar pair to the Thom(p)son Twins?  I need to do a bit of digging)
Of course as already mentioned The Jackson Twins from the newspaper strip.
The Wonder Twins from Super Friends.

[mopee167]

Well, there were the Trigger Twins from DC's 1950s All-Star Western. Each Trigger Twins episode revolved around Walt messing something up, and his brother Wayne, pretending to be the town sheriff, bailing him out.

[Matropolis]

Yank and Doodle - twins who are powerful and invulnerable when near each other but have no superpower when apart

[The Australian Panther]

WOT, nobody nominates DC's 'the Wonder Twins?' They originated in the Saturday AM Superfriends cartoons. I suspect Andrew at least would remember them. [No, Paw beat me there]
They were recently revived and given a series of their own courtesy of Brian Bendis, who, judging by the properties he took on at DC, grew up on this stuff.
https://comicvine.gamespot.com/wonder-twins/4060-56901/

Zan and Jayna. Alien twins from the planet Exxor. When together,the two can use their unique shapeshifting powers. Jayna has the power to transform into any living creature, and Zan into any non-Living organism.

Did not know they were featured in Smallville. One of these days I'm going to have to BingeWatch that show.
In WandaVision, apparently she has given Birth to twins, as in the orignal comic story, and they have Superpowers.
https://www.cbr.com/watch-wandavision-twins-recreate-thors-swing-at-captain-america/
Gold Key had Dono and Koko the Jungle twins.
In France, Georges Bess and Alejandro Jodorowsky put out a two issue miniseries, 'The Magical Twins'
Villains - In the X-men we have the Fenris Twins.

The twins Andreas and Andreas Von Strucker are the children of Wolfgang Von Strucker, the leader of Hydra in the Marvel Comics universe.

And these two are very perverse characters indeed.
Maybe not in 'the gifted'
https://comicbook.com/marvel/news/the-gifted-cast-x-men-marvel-fox/#5
For Paw Broon and I, it always comes back to the Phantom, and the Phantom's twins, Kit and Heloise are now in their mid-teens and both very strong characters in thier own right.   
and then there is always Sugar and Spike.
And no Andrew, no brain meltdown for me, just very good with search engines and triggering memories. Very relaxing actually!
Cheers!   

[SuperScrounge]

and then there is always Sugar and Spike.

Wait, what? Pretty sure they were unrelated.

Fighting American was a twin and his brother's murder encouraged him to become a superhero.

Kobra was a villain with a twin who had a physical telepathy (if one is hurt the other feels it) this lasted the whole run of Kobra and a Batman story in Five Star Super-Hero Spectacular whereby Kobra found a way to kill his brother without suffering the fatal consequences.

Crimson Fox a superhero who mainly appeared in Justice League Europe. The superhero identity was used by twins who were raised separately so sometimes she spoke English, other times French. I think one twin was killed at one point and the other twin a few years later.

A webcomic called Masked seems to feature a heroine who's twin sister is a supervillain. Not 100% sure as the writing is a bit wonky and hard to get into.

Clones aren't technically twins, but... ehh...

The Midwitch Cuckoos in the Marvel X-books, I believe were clones of Emma Frost.

In the mangas A Certain Magical Index and/or it's spin-off A Certain Scientific Railgun the Sisters were clones of Accelerator and were part of an experiment to create a Class 6 power (Class 5 being the highest in that world).

[Andrew999]

Should we include biomorphs like Duo Damsel?

I like the idea of Masked - one twin a heroine and one a villain - that's an idea that appears underused. Was there a suggestion at some point that Professor X and Magneto might have been twins? I might be misremembering that - I was always more of a DC fan and read little Marvel

[mopee167]

The funny thing about Yank and Doodle (mentioned by Martropolis above) was that their other identities were Rick and Dick Walters, tho it was never made clear which was which. And aren?t both Rick and Dick used as common nicknames for Richard???

[profh0011]

NOT in comics, but, I was just relatng this story the other day...

Back in the 80s, the place I worked, there were 2 sisters, Karen & Barbara.  Karen was there first, Barbara was hired later.  They were twins.  Karen had straight hair, and a deeper voice, Barbara had curly hair, with a higher voice.  But after awhile, I was able to tell them apart, JUST by looking at their faces.  Nobody else n the place seemed able to do that.

One day... they came in.. and had SWAPPED hairstyles.  I noticed it immediately, and found it hilarious.  Since it wasn't just one or the other, but both at the SAME time, it had to be a joke they'd conspired to do.  But they REFUSED to admit it.   

[Andrew999]

I have two twins stories:

1) I used to have an office manager called Jackie. I vaguely knew she was a twin. One day I walked into the office pool and saw Jackie talking to one of the other staff. 'Oh Jackie,' I said, 'could you .....' only it wasn't Jackie - it was her twin sister who'd popped in to pick her up. It felt really odd, because I'd only ever seen the one twin - every day for nearly two years - and now suddenly, there was an identical being standing in front, dressed exactly the same, but who was a different person. It felt like something from Invasion of the Body Snatchers

2) For years, I didn't realise my local newsagent was run by two elderly identical twins. I must have had dozens of conversations with what I thought was the same person. Then one day, I walked in and there were two of them behind the counter - talk about being freaked out.

[Robb_K]

As far as Human character twins, I can only remember Fox Features' O'Brine Twins (Waterman and Wetmore), who were silly and a bit mentally slow sailor, during the early and mid 1940s, and The Applejack Twins (Slappy  and Happy), who were mischievious Appalachian country hicks, who seem to have been patterned partly after the pair of twin hicks, Lem and Luke Scragg, in Al Capp's "Li'l Abner" strip (at least being not very intelligent, hicky and gap-toothed - but not as huge or ugly as The Scraggs). 

For my area of expertise, I can list many, many funny animal character sets of twins (and a few sets of triplets), including Disney's Donald Duck' nephews, Huey Dewey, and Louie Duck, and Daisy's nieces, April, May, and June, as well as Carl Barks' Beagle Boys (quintuplets, sextuplets, septuplets, or even, possibly octuplets)(it is debatable, and has been, more or less, unlimited in depictions in story panels.  There were many, many more sets of twins and triplets in The Disney universe (including Lady and The Tramp's Scamp and his 3 sisters). 

Speaking of Disney Comics, I, myself, invented Lucky Gladstone Gander's long-lost identical twin brother, Sadstone", who inherited all the "bad luck genes" in their shared egg, when Gladstone inherited all the "good luck genes".  And so, Sadstone ran away from home many years before they became adults, to become a hermit, dwelling in a cave well-hidden in remote mountains, to avoid having his bad luck's disasters do any more harm to his family members, and to the other fellow Duckburgians.  Unfortunately, though my editors loved the story's premise, and the possibilities for Saddy's future use as a recurring character, they couldn't accept his being such an extremely close relationship, "because the readers would question why he was never seen before this story (despite my logical explanation (which WAS used in the published story).  They accepted the story, and paid me, but only upon my agreement to allow them to change Sadstone's relationship to Gladstone as an "identical first cousin" (as Patty and Kathy in 'The Patty Duke Show').   So, we have an "almost" pair of comic book twins here.

All the other US major Animation studios: Warner Brothers (The early 1940s monkeys - Flip and Dip (fraternal twins), Beep Beep the Road Runner's 3 sons,  MGM's Barney Bear's two nephews, Fuzzy and Wuzzy, and Walter Lantz's Woody Woodpecker's two early 1940s nephews (Splinter and Knothead), who in the late 1940s morphed into fraternal twins, and Oswald Rabbit's twin nephews(Floyd and Lloyd).

Felix The Cat had a pair of twin nephews(Inky and Dinky) in his Harvey comic book stories.  While with Toby Publishing, he had only one nephew, Inky.  Also, Harvey's Rags Rabbit, had two twin nephews(Pesty and Jesty) during the entire 1950s.

Avon's Harrison Cady's(and later, Vince Fago's) newspaper comic strip reprints of Peter Rabbit(#1-7) featured Peter having a pair of young twin sons (Buddy and Petey, Jr.; and Uncle Wiggily also had a pair of twin nephews in their comic strips, and Dell comic books, as I recall. 

And, Rudi Dirk's Katzenjammer Kids (Hans und Fritz) were twins, as well.

MLJ's Top-Notch Laugh Comics had The Three Monkey-Teers, who were triplets, during the early to mid 1940s.

ACG's  "Ha Ha Comics" had Izzy and Dizzy, who were Papa Bear's twin sons, who used Aladdin's magic lamp to grant their every wish. 

The simpleton almost adult twins, Tweedle Dum and Tweedle Dee, had their own, regular feature in a few different regular "Alice in Wonderland" series (Ziff Davis)(Disney), plus as sometime guests in a few fairy tale
series.

I'll add more as I remember them.

[SuperScrounge]

Warner Brothers

Oh, man, that reminded me of an in-joke from Omac (not Kirby's version, but the attempted continuation that ran as a back-up in DC's The Warlord). One of the future corporations was Verner Bros. run by triplets Werner, Turner, & Vernon.

Later on in the pages of Blue Devil, Verner Bros. was the present day film studio Dan (Blue Devil) Cassidy worked for, but I don't remember if the present day brothers were twin/triplets or not.

[profh0011]

I have two twins stories:

1) ...suddenly, there was an identical being standing in front, dressed exactly the same, but who was a different person. It felt like something from Invasion of the Body Snatchers

2) For years, I didn't realise my local newsagent was run by two elderly identical twins. I must have had dozens of conversations with what I thought was the same person. Then one day, I walked in and there were two of them behind the counter - talk about being freaked out.

DRESSED EXACTLY THE SAME?  I wonder if they weren't pulling a joke like Karen & Barbara.

As for the other story... hilarious.  That's never happened to me... THAT I KNOW OF.   

[Captain Audio]

What about the 5 (perhaps more?) identical brothers that showed up now and then on Agent's of SHIELD.
Never watched the series closely enough to get the full skinny on those guys.
They did seem remarkably unconcerned by the loss of a brother.

[The Australian Panther]

Being a Marvel show, that was probably the mutant Madrox the multiiple man. He is always creating copies of himself, sometimes he completely loses track of them and they go off and have separate lives for a time. Many of them get killed.
He was [or one of his duplicates was] an agent of Shield at one time.
James Madrox (S.H.I.E.L.D. Agent) (Earth-616)
https://marvel.fandom.com/wiki/James_Madrox_(S.H.I.E.L.D._Agent)_(Earth-616)
I really like the Character. He has many possibilities. Pun intended.
Has had his own series. 
Actually this would be a great superpower. I'd get one to Study, one to do housework, send one out to work.....'!
cheers!   

[Captain Audio]

Being a Marvel show, that was probably the mutant Madrox the multiiple man. He is always creating copies of himself, sometimes he completely loses track of them and they go off and have separate lives for a time. Many of them get killed.
He was [or one of his duplicates was] an agent of Shield at one time.
James Madrox (S.H.I.E.L.D. Agent) (Earth-616)
https://marvel.fandom.com/wiki/James_Madrox_(S.H.I.E.L.D._Agent)_(Earth-616)
I really like the Character. He has many possibilities. Pun intended.
Has had his own series. 
Actually this would be a great superpower. I'd get one to Study, one to do housework, send one out to work.....'!
cheers!

Nope, the character(s) I'm thinking of are the Koenig quadruplets, actually four brothers and a sister all but one brother having Shield connections.
Eric was murdered by Grant Ward and the brothers and sister later become involved.
https://marvelcinematicuniverse.fandom.com/wiki/Eric_Koenig#:~:text=Expert%20Technician%3A%20Eric%20Koenig%20was%20a%20S.H.I.E.L.D.%20agent,readings%20more%20than%20his%20suspicions%20towards%20Grant%20Ward.

There's a lot of speculation about their true nature, possibly clones of their grandfather who also looked just like them.

[SuperScrounge]

Marvel had The Collective Man, Chinese quintuplets who could merge into one being and split apart.

[Matropolis]

The funny thing about Yank and Doodle (mentioned by Martropolis above) was that their other identities were Rick and Dick Walters, tho it was never made clear which was which. And aren?t both Rick and Dick used as common nicknames for Richard???

That is very true. I couldn?t tell you who is who either! Perhaps in some instances it doesn?t really matter or maybe they were different colour clothes to tell them apart.

[jrasicmark]

I can verify SuperScrounge comment since I'm a fan of Sugar & Spike. They definitely are not twins, they were next-door-neighbors.

[jrasicmark]

As far as Human character twins, I can only remember Fox Features' O'Brine Twins (Waterman and Wetmore), who were silly and a bit mentally slow sailor, during the early and mid 1940s, and The Applejack Twins (Slappy  and Happy), who were mischievious Appalachian country hicks, who seem to have been patterned partly after the pair of twin hicks, Lem and Luke Scragg, in Al Capp's "Li'l Abner" strip (at least being not very intelligent, hicky and gap-toothed - but not as huge or ugly as The Scraggs). 

For my area of expertise, I can list many, many funny animal character sets of twins (and a few sets of triplets), including Disney's Donald Duck' nephews, Huey Dewey, and Louie Duck, and Daisy's nieces, April, May, and June, as well as Carl Barks' Beagle Boys (quintuplets, sextuplets, septuplets, or even, possibly octuplets)(it is debatable, and has been, more or less, unlimited in depictions in story panels.  There were many, many more sets of twins and triplets in The Disney universe (including Lady and The Tramp's Scamp and his 3 sisters). 

Speaking of Disney Comics, I, myself, invented Lucky Gladstone Gander's long-lost identical twin brother, Sadstone", who inherited all the "bad luck genes" in their shared egg, when Gladstone inherited all the "good luck genes".  And so, Sadstone ran away from home many years before they became adults, to become a hermit, dwelling in a cave well-hidden in remote mountains, to avoid having his bad luck's disasters do any more harm to his family members, and to the other fellow Duckburgians.  Unfortunately, though my editors loved the story's premise, and the possibilities for Saddy's future use as a recurring character, they couldn't accept his being such an extremely close relationship, "because the readers would question why he was never seen before this story (despite my logical explanation (which WAS used in the published story).  They accepted the story, and paid me, but only upon my agreement to allow them to change Sadstone's relationship to Gladstone as an "identical first cousin" (as Patty and Kathy in 'The Patty Duke Show').   So, we have an "almost" pair of comic book twins here.

All the other US major Animation studios: Warner Brothers (The early 1940s monkeys - Flip and Dip (fraternal twins), Beep Beep the Road Runner's 3 sons,  MGM's Barney Bear's two nephews, Fuzzy and Wuzzy, and Walter Lantz's Woody Woodpecker's two early 1940s nephews (Splinter and Knothead), who in the late 1940s morphed into fraternal twins, and Oswald Rabbit's twin nephews(Floyd and Lloyd).

Felix The Cat had a pair of twin nephews(Inky and Dinky) in his Harvey comic book stories.  While with Toby Publishing, he had only one nephew, Inky.  Also, Harvey's Rags Rabbit, had two twin nephews(Pesty and Jesty) during the entire 1950s.

Great list by Rob_K. But out of all the Disney twins, how could you forget Mickey's nephews, Morty and Ferdie? I read that Minnie also had nieces and sometimes they were twins, but sometimes they were triplets, and sometimes one would be depicted as a lone child with no siblings. Are Disney's Chip and Dale supposed to be twins? Or Warner's polite gophers, Mac and Tosh? Or Terrytoons' Heckle and Jeckle?
You mentioned Woody Woodpecker's nephews, Knothead and Splinter, and I think you are right; they were male twins in their earliest appearances, but at some point, Splinter turned out to be a girl. And I remember a comic book story where Chilly Willy's nephews appeared, though I don't remember their names.
You also mentioned Felix the Cat's nephews, Inky and Dinky (I love their names, and I always thought it would have been cool if they had added a sister named Pinky and a skunk friend named Stinky). Anyway, I sometimes like to look for obscure comic strips in Google's newspaper archives, and I found a very old Felix comic strip that introduced his nephews, but they were quadruplets. And none of them were named Inky and Dinky though I've forgotten their names. I think they stayed with Felix for about a week of gags, but I don't know if they were ever used beyond that.
Now that I think of it, Popeye had quadruplet nephews who all looked exactly like Popeye, only shorter. I don't think they ever appeared in Popeye's comic strips or comic books.
There's also the Flash's 30th century descendants named the Tornado Twins.
Tweedledum and Tweedledee were also Batman villains.
There are twin girls with purple hair who attend Springfield elementary with Bart and Lisa.
In the Charlie Brown Christmas TV special, there were twin sisters who joined the rest of the Peanuts gang in dance. I always thought the animators made up those characters just so there would be more dancers, but a few years ago, I read that they actually appeared in a week's worth of gags in the Peanuts comic strip. Surprisingly enough, their parents named them Three and Four. They said their parents were named One and Two.
There was also an obscure TV cartoon in the 1960s called the King and Odie (the king was a lion who was literally a king and Odie Cologne was his chief advisor; anyway, his nephews were named Duke and Earl.

[jrasicmark]

I just remembered Si and Am the Siamese cats from 101 Dalmatians (or were they in Cinderella?).

[SuperScrounge]

I can verify SuperScrounge comment

Thank you. 

There's also the Flash's 30th century descendants named the Tornado Twins.

Don and Dawn Allen.

There are twin girls with purple hair who attend Springfield elementary with Bart and Lisa.

Sherri & Terri.

In the Charlie Brown Christmas TV special, there were twin sisters who joined the rest of the Peanuts gang in dance. I always thought the animators made up those characters just so there would be more dancers, but a few years ago, I read that they actually appeared in a week's worth of gags in the Peanuts comic strip. Surprisingly enough, their parents named them Three and Four. They said their parents were named One and Two.

Yeah, they were the sisters of 5, who made the most appearances of the 95472 family. (The father cracked up doing paperwork and gave his family numbers and changed their last name to their zip code.) 

I just remembered Si and Am the Siamese cats from 101 Dalmatians (or were they in Cinderella?).

I think it was Lady and the Tramp.

Just remembered Dover & Clover who were twin detectives who took over More Fun Comics from Green Arrow & Superboy.

[jrasicmark]

I thought of a bunch more.

E-man once fought the Entropy Twins in his first series from Charlton. I think they were aliens who actually didn't mean any harm. They recently re-appeared helping E-man in the Charlton Arrow anthology.

Ma Hunkle, the original Red Tornado had two sidekicks called the Cyclone Kids who I think were twins in DC's Scribbly strip. I'm not sure of their real names.

Superman encountered teenaged alien twin troublemakers from Alpha Centauri named Zigi and Zagi. I think they appeared at least twice. The stories were reprinted in a Limited Collectors' Edition.

Then there's Gold Key's Jungle Twins, Tono and Kono.

[paw broon]

In British comics The Beano had The Danger Twins:-
http://www.internationalhero.co.uk/d/dangtwin.htm

[jrasicmark]

How could I forget?
In the George of the Jungle cartoon, George's wife (or was she just a girlfriend? I can't remember) Ursula had a twin sister named Bella.
Though I don't remember Bella appearing in the show much if at all, except in the opening song ("...While Bella and Ursula stay in step...WHOOA!"). And Ursula was in the live action movie remake, but not Bella.
I never got why they even included the character

[jrasicmark]

In British comics The Beano had The Danger Twins:-
http://www.internationalhero.co.uk/d/dangtwin.htm

You just reminded me... didn't the Bash Street Kids include twins named Sidney and Toots?