The gag was probably first played for the comedy: actors who put on dresses and wigs and stand as their anima, their female opposites. Think Adam Sandler. Twins are said to make you laugh.
The motif of crossed, confused twins, however, probably goes back further than even Shakespeare's comedies. If twins weren't the evil antagonists of myths and parables (double, literally: two-sided), they were something abnormal and Kubrick-like: a fear you experienced in a hallway.
However, in recent years we have seen actors take on the dual role of something more serious and narrative. In
I Know This Much Is True in the most recent double role, the twin concept thwarts biblical responsibility and states that Mark Ruffalo does more than just cut his hair. (Apparently there was a lot to eat.)
But whether dramatic or funny, the double role is never not interesting (even if it is a little tricky). Here are 20 actors who shaved, changed, gained weight and shot the same scene dozens of times as different characters. No stunt doubles required.