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 :
Go to the End
Beat the enderdragon
Go near the overworld portal
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
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Confirmed, and duplicate of MC-1607