Most everyone gets the idea this is about biological twins, not look-alikes. Any variety of twin defined in the Twin Wiki qualifies. Other multiples too I suppose.
Now I’m including bonus points for the rarest variety of twinning for any pair of people where at least one has an acting credit. Identical and fraternal twins have already been mentioned. First mention of Monochorionic–Monoamniotic wins the thread.
Martin Sheen has a twin brother, Joe Estevez, who filled in for Sheen in a few scenes of Apocalypse Now when Sheen had a heart attack.
He’s acted in a lot of stuff, but his career trajectory has not been nearly as successful as his brother Martin; at least judging by the fact that several of the movies he’s been in have been given the ‘RiffTrax’ treatment.
ETA: correction— I swear I read somewhere Sheen had a twin brother who fillled in for him on Apacalypse Now, and though he did act as a stand-in on the movie, he’s actually Sheen’s younger brother.
Billy and Bobby Mauch had the title roles in The Prince and the Pauper.
When I googled to get the names right I discovered that the 2000 remake (which I had never heard of) also had twins in the title roles - Jonathan and Robert Timmins.
Matthew I recal from a short stint on SNL then in Duet, one of the first live-action FOX shows that was pretty good. If I remember seeing his brother it was Not Necessarily the News in the early 80’s on HBO.
Kiefer Sutherland has a fraternal twin sister, Rachel, who works in the film industry, but as a post-production supervisor and producer, primarily for TV series.