Wow, he seemed to do that a lot! I just checked IMBD to remind myself who he was (the name itself didn’t ring a bell), and found that besides Bewitched, he played more than one character on Lou Grant, Nanny and the Professor, Gomer Pyle, U.S.M.C., The Beverly Hillbillies, The Many Loves of Dobie Gillis, Pete & Gladys, The Lucy-Desi Comedy Hour, and I Love Lucy, as well.
I was thinking of that but decided not to mention it since she was basically an extra so didn’t fit in my OP. Then they threw in a quick reference to Martha having a cousin who died while working for Torchwood and it then brushes up against the Evil Twin (Or in this case Identical Cousin) rule.
Eve Myles played the psychic housekeeper in the Doctor Who episode with Charles Dickens then they brought her back to star in Torchwood. Probably will wind up being a case of identical ancestors.
One of the best episodes of TV ever. Incredible performances from Andre Braugher and Donofrio. Donofrio played him as a complete tool, totally unsympathetic. And yet he gets you to feel for him by the end.
Them being the same character is just supposition. It was never answered on the show, some people have decided in their minds, but it isn’t known. It was mentioned as such in a comic book I think, but there is great contestion as to whether it is considered cannon.
And all crotchety, too, I’ll bet, God bless him.
And speaking of Three’s Company, you know the opening bit each week? The part where Jack is on a bicycle on a sidewalk or boardwalk at the beach and coming toward us? And a girl in a bikini is walking away from the camera, and he crashes the bike because he was looking at her? That girl was Suzanne Somers in a wig! (So said Joyce DeWitt on Larry King Live.)
Although it might be considered cheating, Sarah Michelle Gellar also played The First Evil during the 7th season of Buffy (while the First also took on the forms of The Master, Drusilla, The Mayor, Adam, Warren, Jenny Calendar, Jonathan, and Principal Wood’s mom, Buffy was the form she took on most of the time), and she also played Faith in one episode (granted, Faith in Buffy’s boddy).
Due South is full of actors being recast as other characters elsewhere. A first season episode features a Canadian mafioso who later gets recast as Constable Turnble, a goofy dumb-as-nails by-the-book Mountie. Another episode, taking place entirely outside of Chicago, recast a subtantial portion of the cast as locals in a small Illinois town.
Similarly, JAG would recast the regulars as different characters for episodes taking place in the distant past. David James Elliot, who stars as Commander Harmon Rabb, Jr., played his own character’s father in at least one episode (easily distinguished by his having a mustache when playing his dad). Catharine Bell notably plays both Lt. Colonel Sarah MacKenzie and one of Harm’s ex-girlfriends (who was murdered), the strong resemblence between the characters being commented on from time to time.
“It sounds like I have a twin out there.”
“Not anymore.”
On Friends, Giovanni Ribisi played the recurring character of Phoebe’s brother, but in an earlier episode I remember seeing him as a random guy who passes Phoebe on the street playing her guitar and throws a condom in her guitar case, then at the end of the episode runs back to her out of breath asking if he can have it back. 
That definitely could have been the same character. Phoebe just didn’t know he was her brother at the time.
I thought maybe that was the case, but the brother lives upstate and Phoebe brings him down for his first visit to the city.
Rachael Dratch’s entire role on 30 Rock was to play a different character each episode. She was originally supposed to play Jenna, but that character got retooled into a dumb blonde, which didn’t fit her style of acting. I don’t recall her being in season 2…so either she was dropped from the show (which I can’t see happening, consider her and Tina Fey are all BFF) or her costumes & acting have improved immensely.
In the Northern Exposure episode about the founding of Cicely (3.23 Cicely), Most of the cast played other characters from the founding. Additionally, there were many dream sequences that had Rob Morrow playing everything from his “Conniving Rogue Twin” to Ratso Rizzo ála “Midnight Cowboy”.
Moonlighting had the famous “Taming of the Shrew” Episode. This probably doesn’t count, but damn that was a great episode.
Jim
The ultimate case of this, I think, is a show I’ve brought up on this board before – Nichols, the 1971-2 James Garner series where he played one character, then, when the ratings started to slip, they killed that character off and brought in James Garner as his own identical twin to avenge the original charater’s death.
In other words, James Garner played two different characters as the lead in the series.
Dan Castanella played two roles on “Married…l.with Children” a gay guy whose husband was going dancing with Peg, and a really bizarre funeral director. Both of them were…excellent.
All this time and the best example of all hasn’t been mentioned. English comedian Chris Langham made a series called Help in which he plays a psychiatrist seeing a number of patients. All of the patients are played by Paul Whitehouse. Langham insisted that Whitehouse do the parts even though they had never worked together or even met.
Very funny stuff in a very British way so maybe never seen in the US.