When i repair a tool with another tool in a crafting table or the personal 2x2 crafting table, the items are duplicated. I discovered this while trying to merge 2 damaged bows then shift clicking the fully healed bow into my inventory when my inventory filled with healed picks.This applies to weapons and tools for sure.
Duplicate of MC-46715 - If you have not, please use the search function in the future, to see if your bug has already been submitted. If you could not find the original report, please comment with the keywords you searched for.