If you go to the End and beat the Enderdragon a portal will create. But if you look to south (f=0), a ghost portal will create. The ghost portal move with you.
This also happens with hacked overworld portal (e.g. edited with Mcedit or other world editor).
How to reproduce :

1. Go to the End
2. Beat the enderdragon
3. Go near the overworld portal
4. Look at the south

There is two screenshot
Number 1 is a unique ghost portal after kill the enderdragon
Number 2 is two ghost portals
On the left the portal is the enderdragon's portal
On the right the portal is hacked with Mcedit