Put the summary of the bug you're having here

What I expected to happen was...:
Firstly I was expecting that when I defeat the Dragon, the portal blocks and the dragon egg will spawn in the portal in the middle of the island.

What actually happened was...:
The game creates another full version of end portal to leave the end on top of the exsisting one.

Steps to Reproduce:
1. Open a portal to the End
2. Fight the Ender Dragon
3. Kill the Ender Dragon