Summary:

When you enter the end portal and instantly exit the world after loading, 1 closest crystal is duped and 2 end crystals will stand on the pillar closest to the player.

Steps to reproduce:

1. Create a new world/enter a world in which you were not in the ender world.
2. Enter the end portal.
3. After loading, immediately click "Save & Quit".
4. Entering the world again.
5. Go to the nearest obsidian pillar.
6. Look at the amount of end crystals on the pillar.

Observed results:

On the 1 pillar closest to the player there will be 2 end crystals instead of 1.

Expected results:

Each pillar should have 1 end crystals.

Notes:

Also, duplication of end crystals (not only 1 first one) can be obtained after visiting the center of the island, after reloading the world. However, it happens randomly.