Summary: Using the /entitydata command to change these Data Tags on Zombies from: HandDropChances:[0:0.85f,1:0.85f] or ArmorDropChances:[0:0.85f,1:0.85f,2:0.85f,3:0.85f] to: HandDropChances:[0:2f,1:2f] or ArmorDropChances:[0:2f,1:2f,2:2f,3:2f] and killing said Zombies will cause them to drop weapons/armor that are always undamaged, if they have any, no matter how many times you try. This might also be the case for Skeletons/PigZombies.

Instance: After reproducing/confirming MC-96526, I spawned a Zombie to kill all of the Villagers (I had about 200). Some had weapons/armor. I wanted to modify their Data Tag, so that they would always drop their equipment. I didn't know the Data Tags at the time so I did: /entitydata @e[type=Zombie] <PersistenceRequired:1>. I then read the entity data in the chat and found the Data Tags that controlled the chances of equipment dropping. I used this command: /entitydata @e[type=Zombie] <HandDropChances:[0:2f,1:2f]ArmorDropChances:[0:2f,1:2f,2:2f,3:2f]>. When I killed all of the Zombies, I looked at the equipment and found all of
it undamaged. I kept testing and there were no cases where equipment was undamaged and deducted it was a bug, I am not sure if this is a WAI.

How to Reproduce:
1. Spawn lots of Villagers, I recommend you contain them to prevent any escapees.
2. Set difficulty to hard, make sure it is night.
3. Spawn some Zombies and wait till all of the Villagers are infect.
4. Use this command: /entitydata @e[type=Zombie] <PersistenceRequired:1,HandDropChances:[0:2f,1:2f]ArmorDropChances:[0:2f,1:2f,2:2f,3:2f]>
5. Kill all of the Zombies. (Preferably with /kill @e[type=Zombie])
6. Repeat if none or few weapons/armor is dropped.