The same actors playing different parts in sitcoms or dramas

A Nero Wolfe Mystery did this deliberately to initially-confusing effect, with the guest parts in each episode played by the same repertory cast.

Arthur Lowe played both Captain Mainwaring and his brother, Barry, in an episode of Dad's Army.

Bewitched, with Liz Montgomery playing her regular role of Samantha & her 'guest role' of cousin Serena.
Similarly Barbera Eden played her regular role of Jeannie in 'I Dream Of Jeannie' & her 'guest role' of her evil sister!

I think it's more common than people realise, especially in soaps. Actors get a small part in a handful of episodes then a few months or years down the line are cast as a full time character. Rob Norbury who played Riley in Hollyoaks for the last couple of years had a few years earlier played a Sixth Former, while the actor playing Billy in Shameless previously had a one episode stint as another character as a younger kid.

small example of this was in home and away christie hayes played both kirsty sutherland and her real twin as story wise the twin she thought was her twin ,wasn't her twin but had been swapped at birth for another twin who just that just happened to be identical ideal for her to play, think this lasted 2 or 3 episodes just before she left the series however.

Several character actors returned in several eps of Hancock's Half Hour. Hugh Lloyd was a librarian (The Two Murderes), a TV repair man (The Set That Failed), a council tree marker (The Tree), and most notably, a blood donor who walked off with Hancock's wine gums (The Blood Donor). John Le Mesurier (aka Sgt Wilson) was a judge (The Crown vs S. James), a doctor (The Cold), the chairman of the National Trust (Lord Byron Lived Here) and "an intrepid bird man" (The Lift). John Vyvyan, Mario Fabrizi and Peggy Mount were also regular guest stars.

Others I can think of are Eve Myles (Dr Who S1E2, Torchwood) and Terence Conoley who played Mr Waring in the first episode, "A Touch Of Class", of Fawlty Towers as well as Mr Johnston in "Waldorf Salad".

This happened a lot on Murder She Wrote. The same actors would appear as different characters every few years, but I guess because MSW went on for so long it wasn't really a concern. Similarly, The Golden Girls also re-used guest stars occasionally in different roles.

And another example of one actor playing two roles within the same episode would be Lisa Kudrow in Friends - she played both Phoebe and her twin sister Ursula in some episodes.

Seems to happen in the Buffyverse too:

Brian Thompson as Luke (vampire lackey of the Master) in Episodes 1 and 2 of Season 1, plus The Judge in Season 2

Eliza Dushku played Buffy and Sarah Michelle Gellar played Faith in "Who Are You" (season 4)

Kali Rocha played Spike's love Cecily in Season 5's "Fool For Love", and later played the vengeance demon Halfrek in Season 6 and 7, although it's briefly intimated that the characters are one and the same.

Tom Lenk plays Cyrus, a vampire minion, in Episode 2 of Season 5, as well as Andrew Wells in Seasons 6 and 7.

Not an uncommon practice in episodic television.

James Sloyan played a range of alien and human characters in Star Trek TNG, also Deep Space Nine and Voyager. I think he may have also played a Bolian in one of the Trek movies.

Similarly British actor Carolyn Seymour played four different guest roles in the 7 series of TNG, and Michelle Forbes had a very impressive cameo in Next Gen before being offered the regular role of Lieutenant Ro Laren.

That's just one TV franchise.

Casting directors will always want to work with good actors again and again; and how well they fit in with the regular cast and production team has a large influence on the decision.

Actors are, by the nature of the job, supposed to be able to play different parts; so its unsurprising that talented ones are rehired to do exactly that.

That wasn't really the same thing though. In the show the characters had swapped bodies.

Ergo they were different characters.

I think Jean Marsh appeared on The Saint 4 times playing different characters (or the case of ITV4 showing the colour episodes, every couple of weeks).
http://www.imdb.com/name/nm0550577/

Annette Andre was also on it twice and Kate O'Mara 3 times.