Ender Dragon on Overworld.

I was in The End, the ender dragon killed me,I respawned in my bed near stronghold and i jumped into the portal,I was killing the ender dragon and she push me into the portal, so once again I respawned in my bed near the stronghold, one more time I entered into the portal and when I spawned in The End the dragons wasn't there.
I invited my cousin to play in my world, he had never entered to my world,  and whemn he did it, The Ender Dragon was in the  Spawn.
Please Fix it, many people had their house near the spawn, The ender Dragon broke all the blocks.