You can clone physical appearance, but not everything else.

Just ask them both a really personal question like... "Which jam do you prefer? Raspberry, or Raspberry Seedless?"

Then, like in all the films, the clone will say the wrong answer, get shot, whimper about "How did you know?!" and be told only such and such knows that him over there prefers this Jam to that.

Simple, really.

EDIT: Or there's the Monty Python 'Bridge of Death' approach.

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We may be able to clone animals (Dolly the Sheep), but methinks we can't really clone humans yet... well, we can, but not without all the super-aging, and arthritis and all that.