Mirror Test

The world record of this going on was three whole days before the guy noticed that was his double from the future, not a reflection in a mirror.

When someone sees an exact replica of themselves and believes that there is a large mirror in front of them causing a reflection. For the joke to work, the double also needs to believe there is a mirror in their way. The "only" way to see that if there's a mirror is doing a bunch of unpredictable things, like patting themselves on the head or rubbing their belly, instead of reaching their hand out and trying to feel a mirror. This joke either ends with one of the doubles attacking the other or the two simply dropping it.