Items duplicate when you put 2 items together to repair them. i.e. 2 damaged bows give you unlimited bows. For swords, bows, pickaxes, axes, hoes, and all armor.
Linked issues
duplicates 1
Comments 2
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.
Confirmed, noticed this when repairing a bow when my sister and I were playing LAN minecraft on the current snapshot, 14w05b.