The bug

When you Ctrl middle click on a tile entity to copy the NBT tags of it you receive a new item every time even if you have an item with the exact NBT data already in your inventory.

How to reproduce

1. Place a sign and write something on it
2. Ctrl middle click it twice
   → You will receive two sign items in different slots
3. Open your inventory and try to stack the sign items
   → You can stack them which means their NBT data is identical

I rather expect is that the second time you Ctrl middle click the sign, the sign that is already in your inventory is selected