Minecraft Version:1.10.2
Inventory Tweaks Version:1.6.1-68

Testing around with the mod and it appears in the bottom right hand inventory slot (above the 9 inventory on HUD) if you click on the slot then quickly press r to sort inventory it duplicates the item INFINITELY as many times you want ie i had 64 repeaters and ended up doing the glitch once with 128 this will need fixed

EDIT: sometimes it will delete that item from your inventory rather than dupe it

See response to #390.

This is most likely caused by some other mod (or a modded server), just exposed by inventory tweaks.

There have been historically a few times where creative-mode players get unintentional item duplication, but those don't matter because creative-mode lets you duplicate things freely anyway.

Ah, yes you are right this is not an issue for Survival