Migrated from: https://dev.bukkit.org/projects/shopkeepers/issues/527

Originally posted by FatherWh0 (May 20, 2017):

There's a duplication glitch in the editing gui for player shopkeepers. The conditions to make it occur are rather specific. The player shopkeeper needs to be the trade type and have it's trades set up and ready. The player's action bar must be fun. It's best to test it with an empty inventory other than the action bar being full as it is important that none of the items you are attempting to dup can already be in the player's inventory. With this arrangement, open the player shop's editing gui and attempt to shift-click the items into your inventory. You will find the items will appear in your inventory, temporarily disappearing from the player shop's trade gui. When you close and reopen the gui the items will reappear.  I included a video demonstrating the duplication process hopefully with enough detail to duplicate it.
I originally tested this with v 1.81 but then upgraded tot he current 1.82 but the duplication glitch continued to exist.
Video demonstration: https://youtu.be/P-cxNTzQTcc
config.yml https://pastebin.com/5T8D16wX
Shopkeepers 1.81 (and 1.82)
paper 1.11.2 #1101

Originally commented by FatherWh0 (May 28, 2017):

Thanks for your attention to this. I apologize it's taking me so long to respond. Real life is interfering but I will do tests on my test server as soon as I can. There are some plugins that get involved in the inventory so I know what to try first, lol. 
Edited May 28, 2017

Originally commented by FatherWh0 (Jun 4, 2017):

As you suspected the issue was caused by interference from another plugin, StackableItems. It allows superstacking inventory items.
Sorry to bother you and thank you for your time on this.

Originally commented by blablubbabc (May 26, 2017):

I am not able to reproduce this.. The items do not get moved into my inventory on my testing server. Could you try it on a fresh spigot server without any other plugins interfering? And also try it without mods first.
 
The expected behavior (and the behavior I see on my 1.11.2 testing server with shopkeepers 1.82) is that the items might look like they temporarily move into the player's inventory, but they are never actually there. They visually disapear a moment later again.
I don't know why they stay in your inventory and you can move them around into the chest etc. Looks like something (ex. some other plugin) might be interfering.
Edited May 26, 2017

Originally closed by FatherWh0 (Jun 4, 2017)

Originally commented by blablubbabc (Jun 4, 2017):

Thanks for letting me know.