I ([Mod] Pokechu22) have written a tool that can recover your world by removing the duplicated block entities:
- For safety, make a complete backup of your world (this tool modifies files in-place).
- Download this program (source code). Make sure your browser saves it as a .jar file, not a .zip file.
- Run it. A window should open.
- Select "add files", and then find your world, enter the region folder, and select the mca file with the duplicate block entity. If you don't know which file that is, just select them all.
- Wait while the tool runs. Any duplicate block entities will be output in the log (full NBT); other chunks will just have counts.
- The world should now load ingame.
If you have any further information about this bug, please comment.
I thought that using structure blocks would allow me to copy-paste buildings, and it did. However, now that I've done it, opening the map crashes the game. There was another folder created (that wasn't there before) that had a weird symbol in front of the name, and I deleted that after it started crashing.
I honestly don't know if it can be reproduced, and I just want this world back. I hadn't backed it up in a while, and it was really important to me.