When you put 2 pieces of the same type of diamond armour in a CraftingTable the result you get is 1 back.
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You aren't giving enough detail... More than likely not a bug.
When you put two of the same tool or armor in a Crafting Table, then "create" the item to the side, you are repairing. Minecraft's repair was added a couple versions ago. Durability from both pieces combine to make a higher durability piece of armor.