There are lots of reasons why an actor might double up on roles. When you get to work with a face as versatile as Benedict Cumberbatch's, it makes a certain poetic sense to let him play the good guy and the bad guy.
Other times, though, an extra gets promoted almost on the spot. When a small part opens up due to a sudden case of casting regret, they need to throw a warm body in there, continuity be damned.
Here are a few cases of actors clocking double overtime that you may have missed: