i went to combine two bows for full durability so i shift clicked and i filled up my inventory with fully repaired once
What I expected to happen was:
it would get rid of the old bows and give me a new one
What actually happened was:
it duplicated the items

step by step

1: go in game mode 0
2: take two damaged tools/weapons and put them in the crafting slot to combine.
3: take the new tool/weapon or shift click
if you've followed the steps correctly you can duplicate your tool/weapon