You store something force shut down Minecraft then reopen then you have the same amount in the Hot bar and the storage block.

I found it by accident and I thought you guys (Mojang) might want to know.

How to reproduce.

1. First get a storage block (chest, barrels, etc.
2. Put  items you want to duplicate 
3. Use the command  Ctrl + Shift + Esc 
4. Close Minecraft 
5. Go back in and a crash or error will pop up
6. Go back to the same world 

Then you should have a duplicate of the same Item.

Why you should fix it because this might affect how survival games or stuff like that. If people use this then that is basically cheating.