You borrow a coin from a spectator. You ask the spectator to sign the coin, on both sides, to verify that it is their signed coin that you are using. You show the spectator a balloon and ask them to place their signed coin inside the balloon. You then inflate and tie off the balloon. For effect, you can shake the balloon to verify that the signed coin is still inside. You then proceed to do the impossible; you start to pull out the spectator’s signed coin from inside the balloon. The spectator will see the coin emerging from the balloon. You then show that, in fact, there is no hole in the balloon for the coin to emerge from. As an additional kicker, you can burst the balloon and then show it repaired.
No duplicate coins
Start clean, end clean