Stunt Double

If Stunt Double somehow enters the battlefield at the same time as another creature, it can’t become a copy of that creature. You may choose only a creature that’s already on the battlefield.

You can choose not to copy anything. In that case, Stunt Double enters the battlefield as a 0/0 creature, and is put into the graveyard immediately (unless something else is raising its toughness).

If the chosen creature is copying something else (for example, if the chosen creature is another Stunt Double), then Stunt Double enters the battlefield as whatever the chosen creature copied.