[QUOTE=Priceguy]
You know, I used to wonder how they managed to find someone that looked that much like Derricks without actually being Derricks (I’m thinking primarily of “The King Is Back”). This explains it. It does bother me that they’re not identical, though. All the other doubles are.
[/QUOTE]
How do you know they’re not identical? Wikipedia seems to think they’re identical.
[QUOTE=Quartz]
To extend the thread, have there ever been non-identical twins in the same film?
[/QUOTE]
The Olsen twins have made several movies together, and are non-identical.
More celebs with twins:
Mario Andretti
Jerry Falwell
Jerry Hall
Billy Dee Williams
Science-fiction author and physicist Gregory Benford is an identical twin.
[QUOTE=Priceguy]
Renee O’Connor (Gabrielle on Xena) played her daughter, identical to herself, if that counts.
[/QUOTE]
To clarify: both Renee and Lucy Lawless played multiple roles on Xena, though none were explicitly twins.
However, in the same show Ted Raimi played not only Joxer, but also Joxer’s triplet brothers Jett and Jace. I assume that counts for the OP’s criteria.
Bruce Campbell played Good Ash and Bad Ash in Army of Darkness (and an army of mini-Ashes, too).
[QUOTE=Scupper]
Bruce Campbell played Good Ash and Bad Ash in Army of Darkness (and an army of mini-Ashes, too).
[/QUOTE]
Good? Bad?
[QUOTE=cochrane]
And also, the EMH on the Enterprise-E in “First Contact.”
Sorry, twickster. 
[/QUOTE]
And for Og’s sake don’t mention this episode.
Sorry twickster :o