Blocks can be duplicated by exploiting a sub-chunk regeneration glitch in the End

The Bug:

Blocks and block entities can be duplicated by exploiting a sub-chunk regeneration glitch in the End. 

Steps to Reproduce

If you are in survival mode, be careful not to die by falling.

1. Enter the End dimension and kill the ender dragon if the ender dragon is there.
2. Make your way to the coordinates X = 0, Y = 1, Z = 0, which is located below the exit portal on the central End island. After this step, you should be standing on X = 0, Y = 1, Z= 0, with a solid block underneath you.
3. Make a bridge out of any solid block, preferably a solid block that doesn't allow mobs to spawn on it, such as glass.
4. Bridge to the coordinates X = 200, Y = 1, Z = 0. After this step, you should be standing on X = 200, Y = 1, Z = 0 with a solid block underneath you.
5. Place the blocks of your preferred choice while being as near as X = 200, Y = 1, Z = 0 as possible. I would recommend placing the blocks on the top face of the solid blocks you have placed earlier.
6. Save and exit the world normally.
7. Reload the world.
8. Remove the blocks that you want to duplicate, and remove the bridge up until coordinates X = 126, Y = 1, Z = 0.  After this step, you should be standing on X = 126, Y = 1, Z = 0 with a solid block underneath you.
9. Save and exit the world again normally.
10. Reload the world again.
11. You shall now have duplicated your chosen blocks. You may wish to travel to X = 200, Y = 1, Z = 0.

Notes:

If done on block entities, when they duplicate, they will have no block entity data, which could cause further issues like MCPE-188674. This was private because this was a block duplication exploit.