Good movies where actors played dual roles...

Moon Over Parador with Richard Dryfus
Dave with Kevin Klein...Though I don't think Dave and the president were ever shown face to face.

Here are some older ones:

"Stolen Life [British](1939)" starring Elisabeth Bergner which was remade into
"A Stolen Life (1946") starring Bette Davis.

Bette Davis did two movies where she played twin roles, the above and "Dead Ringer (1964"), which was a remake of "The Other One [La Otra {Mexico}](1946)" with Dolores del Rio as its star. The same material was remade in 1986 as "Killer in the Mirror" starring Ann Jillian.

Bad Company - Chris Rock (though the two characters are not on screen at the same time, he plays a CIA agent who is killed on an undercover mission to get a nuclear bomb, and then his gangsta twin brother from Harlem has to take his place and become a CIA agent)

The Nutty Professor - Eddie Murphy plays the entire family!!!

Twin Dragons - Jackie Chan (two babies switched at birth one is an orchestra conductor and the other is a street fighter)

The One - Jet Li (especially the ending where you see Jet Li fighting against Jet Li for the future of the universe)

Jake Gyllenhaal's Enemy.
Dead Ringers.
Moon.

Cloud Atlas.