Keeping entity that already exists with UUID - Survival

Hi, first, sorry for my bad english.

I have two worlds in minecraft 1.9.2 vanilla, after a few minutes the launcher console begins to show too many messages like "Keeping entity that already exists with UUID" and the world begins a little laggy.

i try to teleport to the entity with the command "/tp @e <UUID>" and when the game teleports me, too many of the entities of the world teleports to that place and the game is very laggy in this point

after that i kill those entities with the command "/kill <UUID>" all of those entitites die.

this solve the problems for a few minutes, after that, the same message appears in the console and the same happens.