Ender Dragon respawn after leaving and coming back in the game

after having killed the ender dragon, i've left the game and came back later on, 

I expected to be in the end surrounded only by endermen.

instead I found a new ender dragon

Steps to reproduce:

1. Kill the ender dragon

2. leave the world while staying in the end.

3. come back in the world, and you found the freshly respawned ender dragon.