In Moon by Duncan Jones, there are several scenes where Sam Rockwell
appears as both clones of Sam Bell.
And in particular, there's a ping-pong scene:
How were these scenes shot? Was it a body double, multiple takes, or computer graphics? I doubt it was the latter, since the film's budget wouldn't even allow for CGI to be used for lunar Spice harvesters rovers.
According to a Q&A session with the director there were various methods used.
This article explains:
There were four ways of filming Sam: one was to use a body double, acting alongside Sam; two was to film Sam doing one half of the conversation with a static camera, then shoot again doing the other half with the camera in the same place; three was to use a robotic camera capable of following the same path multiple times to the same schedule, shooting the scene twice then putting them together in post-production, removing/replacing bits of Sam as and when appropriate; four was a mixture of the previous three to allow one Sam to touch the other.
To pull off the effect of one Sam touching the other, they would shoot the scene with Rockwell’s arm tied behind his back, leaning his shoulder against a tennis ball on the wall as a mark point. Then they’d shoot the scene again with him in the second place, his body double in the first place, the body double’s shoulder leaning against the tennis ball but actually doing the touching action. In post, they would then splice the double’s arm onto Sam’s body, giving the illusion of the two clones touching.
