I'm trying to compile a mod https://github.com/gegy1000/Terrarium, it is not mine. It keeps failing with:

* What went wrong:
Execution failed for task ':deobfMcMCP'.
> java.util.zip.ZipException: duplicate entry: net/minecraft/command/AdvancementCommand$Mode.class

 

Running with --debug and going to the relevant output gives me:

 

19:56:40.686 [DEBUG] [org.gradle.api.internal.tasks.execution.SkipUpToDateTaskExecuter] Determining if task ':deobfMcMCP' is up-to-date
19:56:40.707 [INFO] [org.gradle.api.internal.tasks.execution.SkipUpToDateTaskExecuter] Executing task ':deobfMcMCP' (up-to-date check took 0.012 secs) due to:
  No history is available.
19:56:40.716 [DEBUG] [org.gradle.api.internal.tasks.execution.ExecuteActionsTaskExecuter] Executing actions for task ':deobfMcMCP'.
19:56:40.730 [LIFECYCLE] [org.gradle.api.Task] Applying SpecialSource...
19:56:41.064 [INFO] [org.gradle.api.Task] Using AccessTransformers...
19:56:41.080 [INFO] [org.gradle.api.Task] C:\Users\shwes\.gradle\caches\minecraft\net\minecraftforge\forge\1.12-14.21.0.2331\userdev\merged_at.cfg
19:57:09.101 [DEBUG] [org.gradle.api.internal.tasks.execution.ExecuteAtMostOnceTaskExecuter] Finished executing task ':deobfMcMCP'
19:57:09.117 [LIFECYCLE] [class org.gradle.TaskExecutionLogger] :deobfMcMCP FAILED

 

I've tried running it as gradlew build and gradlew clean build and tried it on both windows 10 and debian stretch. there doesn't seem to be any documentation that I can find on it.

Running gradlew test fails with the same error.

Try gradlew cleanCache

That should nuke your cache folder and let it rebuild. However this shouldn't happen at all. YOu may want to try updating your referenced Forge to 1.12.2 not 1.12.