I tried building on Fedora 22 and the latest Ubuntu and get the same error each time:

$ ./gradlew clean build
...
Applying SpecialSource...
:deobfMcMCP FAILED

FAILURE: Build failed with an exception.

• What went wrong:
  Execution failed for task ':deobfMcMCP'.

java.util.zip.ZipException: duplicate entry: net/minecraft/command/AdvancementCommand$Mode.class

• Try:
  Run with --stacktrace option to get the stack trace. Run with --info or --debug option to get more log output.

BUILD FAILED

Same

This is certainly weird -- but I don't see how it could be related to my buildscript. I can't reproduce this on Windows.

Tried it on windows and debian and got the same thing. Can you upload a compiled version somewhere?

Determining if task ':deobfMcMCP' is up-to-date 19:29:42.479 [INFO] [org.gradle.api.internal.tasks.execution.SkipUpToDateTaskExecuter] Executing task ':deobfMcMCP' (up-to-date check took 0.016 secs) due to: No history is available. 19:29:42.494 [DEBUG] [org.gradle.api.internal.tasks.execution.ExecuteActionsTaskExecuter] Executing actions for task ':deobfMcMCP'. 19:29:42.510 [LIFECYCLE] [org.gradle.api.Task] Applying SpecialSource... 19:29:42.869 [INFO] [org.gradle.api.Task] Using AccessTransformers... 19:29:42.885 [INFO] [org.gradle.api.Task] C:\Users\shwes.gradle\caches\minecraft\net\minecraftforge\forge\1.12-14.21.0.2331\userdev\merged_at.cfg 19:30:15.541 [DEBUG] [org.gradle.api.internal.tasks.execution.ExecuteAtMostOnceTaskExecuter] Finished executing task ':deobfMcMCP' 19:30:15.557 [LIFECYCLE] [class org.gradle.TaskExecutionLogger] :deobfMcMCP FAILED

this is the output I get on windows 10 with gradlew build --debug

I've recently made a lot of changes. Does this still occur?

(it's fixed now)

is goooooooooooooooooooood!