Quitting while in The End spawns multiple Ender Dragons: Singleplayer

I made a home in The End out of end stone, put chests, an ender chest, and a crafting table and furnace. Then I quit the game, and later, I joined the game again, and there was more than one Ender Dragon. So I had TWO Ender Dragons to deal with! Then a few minutes later...OMG! THERE'S 4 ENDER DRAGONS! They just kept spawning and spawning, and spawning! The Minecraft screen was black, not because a crash, but because Ender Dragons were blocking the camera! >_<