While making a minigame where you get 1 coin per second, I found a weird glitch. If you're in creative mode, and drop the coins, they appear in your inventory the next second, but are also on the ground. This way you can duplicate any items.

Instructions:
1. Create a 20 tick p/s clock
2. Create a scoreboard objective which gets 1 added per tick (20 a second)
3. Put a command block next to that one which gives all players with a score of twenty 1 item
4. Put a command block next to that which sets all players who have a score of twenty to 0.
5. Throw your items on the ground.