[Crash] "java.lang.IllegalArgumentException: value already present:" - Game crashes when worldgen dimension type presets are not unique

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 only one dimension can use the default values overworld, the_nether and the_end.

How to reproduce

1. Download the worldgen settings

1. 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.

Workaround

Instead of using one of the presets, specify all the default properties like this:

"type": {
  "ultrawarm": false,
  "natural": true,
  "shrunk": false,
  "ambient_light": 0,
  "has_skylight": true,
  "has_ceiling": false
}

Stack trace

20w21a\: https://pastebin.com/j9fPF0vs

Description: Exception in server tick loop
 
java.lang.IllegalArgumentException: value already present: chf@6204b89e
    at com.google.common.collect.HashBiMap.put(HashBiMap.java:287)
    at com.google.common.collect.HashBiMap.put(HashBiMap.java:262)
    at gg.a(SourceFile:51)
    at gg.a(SourceFile:62)
    at gl$a.a(SourceFile:28)
    at net.minecraft.server.MinecraftServer.a(SourceFile:407)
    at net.minecraft.server.MinecraftServer.i_(SourceFile:334)
    at enr.d(SourceFile:65)
    at net.minecraft.server.MinecraftServer.run(SourceFile:666)
    at java.lang.Thread.run(Thread.java:745)