John Lithgow played the “new” High Commander on an episode of 3rd Rock From The Sun
Bart Simpson had an evil Siamese twin in a Threehouse of Horror episode.
Lee Marvin in Cat Ballou was at least his own brother, if not twin. It’s been a while.
You are correct. It has been awhile, although at one time I had this fantasy. . .
Jean-Claude van Damme did it in a crappy movie of some sort… there it is, Double Impact.
Renee O’Connor (Gabrielle on Xena) played her daughter, identical to herself, if that counts.
Richard Sanders played Les Nessman and his mom in an episode of WKRP. Does that count?
Do you consider Alton Brown of the Food Network show “Good Eats” to be an actor?
If so, he has played his twin “B.A.” several times.
Bette Midler and Lily Tomlin played their own twins in Big Business.
Film name from 2006 film: The Prestige.
Just saying that is kind of a spoiler, and saying any more is REALLY spoileriffic.
And if one wanted to get hyper-technical, Nicholas Brendon and his actual twin appeared together in an ep when Xander was split into duplicates, and each actor played both halves, even if only momentarily.
Also from the Buffyverse and again being hyper-technical, Boreanas played both Angel and Angelus, including both appearing on-screen together in one episode.
I know you don’t want soap operas, but Anne Heche is pretty well known outside the soaps; she won a daytime Emmy playing twins Vicky and Marley.
And I can’t believe I’ve forgotten about Twin Peaks before now. Sheryl Lee played Laura Palmer and her near-identical cousin Maddie. And again being hyper-technical, Kyle McLaghlin played Dale Cooper and, briefly, his own evil doppelganger.
Any interest in turning that premise around a little? What about actual twins?
Nicolas Brendan’s twin brother, Kelly, played the second copy of Xander Harris in the Buffy episode The Replacement.
There’s a scene in Terminator 2 Linda Hamilton’s twin sister, Leslie, plays the T-1000 impersonating Sarah Connor. There’s also a cut scene where Linda looks into a mirror while talking to the Terminator - except she’s actually looking through an empty frame at Leslie, who mirrored Linda’s movements so the scene could be filmed without the camera’s reflection getting in the shot.
Then we have Ed, where we have twins Lauren Thomas and Morgan Thomas both playing the part of Sarah Burton. This creeped me out when I found out about it. “You mean that woman isn’t always the same woman?”
I don’t remember that character.
Kids are often played by twins, because of child labor laws – that’s how the Olsens got their start.
Another addition to the Buffyverse hijack (since someone complained about all the Startrek references)…
Amy Acker played Fred on Angel. And she played Illyria–the ancient demon who took over Fred’s body. Illyria was able to “channel” Fred.
It’s said that Fred would have returned in Angel’s 6th season, allowing Acker to play both parts. But Fox decided that 5 seasons were enough…
I’m the OP. I’m allowed to complain. 
George Hamilton, Zorro, the Gay Blade.
Remeber when Mary Kate was shoved into rehab for anorexia a day before her 18th birthday by Ashley and her parents? Probably said they were going to an excercise camp. Man Mary Kate was really played by her twin.
The wife of Mike Burton, Ed’s doctor friend.
Yeah, sure, but this was an adult woman. Well, two, but you know what I mean.
Ew!That was two different women? During the same season? Ew!
Also in Terminator 2 - Don and Dan Stanton played the night guard and his T-1000 clone at the mental hospital where Sarah Connor was held. Looking at their imdb pages - they’ve never appeared in anything separately.