Mobs Can Spawn With The Same UUID As Other Mobs In World

I Used The Command

/summon Zombie ~ ~ ~ {UUID:11111111-1111-1111-1111-111111111111}

And A Zombie With That UUID Was There, But When I Summoned Multiple And Used /kill @e[type=zombie] It Only Killed One, And In The Console It Sayed IT Killed All Of Them. When You Type /kill @e[type=zombie] Again, It Will Say 'Could Not Find Entity' A Bunch Of Times.These Are Not Ghost Mobs, And They Will Attack, Deal Damage,And Get Hurt By Anything.