Duplication of gravity blocks with end gateway

Summary:
With this bug, you can duplicate any gravity blocks. When falling, gravity blocks are pushed by water towards the end gateway and duplicate the block (one remains fallen, the other duplicate is teleported).

Steps to reproduce:

1. Go to the End and kill the dragon.
2. Next to the end gateway that appears, build a small platform as in the screenshot.
   
3. Place a gravity block in the corner that can move the piston.
   
4. Place the sticky piston 2 blocks above the gravity block.
5. Build a redstone clock (using two observers) and connect it to the piston.
6. Wait half a minute and teleport through the end gateway.

Observed results:
Blocks are duplicated.

Expected results:
Blocks will not be duplicated.

Video:
dupe (1).mp4
dupe2 (1).mp4