The bug

When there are multiple dimensions with the same dimension type, the game crashes. When type is a string, it is supposedly a preset for dimension options like ultrawarm, shrunk, etc. However it only one dimension can use the default values overworld, the_nether and the_end.

This is supposedly caused because the game uses the dimension type value in a hashmap. It should instead use the dimension key.

Relevant section in worldgen settings

"dimensions": {
  "minecraft:overworld": {
    ...
    "type": "minecraft:overworld"
  },
  "custom:overworld": {
    ...
    "type": "minecraft:overworld"
  }
}

How to reproduce

1. Download the worldgen settings duplicate.json
2. Create a new world and select the downloaded file when importing world settings
   The game crashes because the custom:overworld also used the overworld dimension type preset.