Entity Duplication Glitch

In my world I have some villagers that are made with commands. They have names, custom heads, custom trades and other tags. I found a EXACT duplicate once. Exact same trades, head, profession etc.
UPDATE:
On 12-3-16, I found 3 of the same villager. Exact same. It duplicated pretty fast becuase I teleport them sometimes and it tells me how many there are.