The enderdragon keep respawning/duplicating

I wanted to go to the end to kill some enderman for the xp and the enderperl and I have already beat the dragon. When I enter in the end the dragon has respawn/duplicate and all the block I have destoyed and place was still there and the portal that take to the overworld too. So everytime a enter in the end or if I leave and reenter my world the dragon spawn again. Some of he thing that heal the dragon reappear for some reason too and they are a little weird looking because I already destroyed them(I put I image of what it's look like).The other image we can see that there is a dragon and a portal that appear after killing him.

some side information: -This is a single player world
-I did put my world in LAN to put myself in creative to make a end portal because I could find the portal ( the ender eye would take me to a strong old but not to the portail and even by searching a lot I couldn't find it so I made one) but nobody joined my world

By the way there is probably a lots of error i'm sorry, I am not a native English.