The bug

A player in Survival or Adventure mode picking up and dropping items modified by the /entitydata command can duplicate the items.

How to reproduce

I made a quick little video, but I had to shove it on dropbox because of the 10MB limit
Link: https://www.dropbox.com/s/bqyjpazljppkirr/2017-04-04%2021-56-30.mp4?dl=0

1. Place a repeating command block, enter for example the following command (you can modify actual item entity tags as well) and activate it

   /entitydata @e[type=item] {SomeCustomOrExistingTag:1b}
   

2. Place different variants of an item (for example different colored wool) in your hotbar
3. Switch to Survival or Adventure mode

   /gamemode survival
   

4. Drop all items in your hotbar by either pressing Q or Ctrl + Q
5. Select the first hotbar slot and keep pressing Q or Ctrl + Q
   → After some time you should see that the dropped items have been duplicated