Situation:
Running the /summon Villager ~ ~1 ~

{UUIDLeast:1,UUIDMost:0}

command twice will spawn two Villagers with both the same UUID.
Expected result:
Running the summon command the second time, the summon command should cancel and inform the player that the Entity cannot be summoned. (Vanilla 1.10.2 behaviour)
Problems:
Because the entities have the same UUID, commands that include the UUIDs of these entities (like /kill @e, /entitydata @e, etc.) do not work as expected, especially after one of the duplicates is killed with /kill)