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:
- Create a new world/enter a world in which you were not in the ender world.
- Enter the end portal.
- After loading, immediately click "Save & Quit".
- Entering the world again.
- Go to the nearest obsidian pillar.
- Look at the amount of end crystals on the pillar.
On the 1 pillar closest to the player there will be 2 end crystals instead of 1.
Each pillar should have 1 end crystals.
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.