Unless you override the jar task completely, any files you specified are
added to the original files, not replaced. To make matters worse, gradle
does not fail jar tasks when duplicate classes are added by default.

We’ve fixed the immediate issue with compiler’s jar file by excluding classes that do not originate from our repackaged/proguarded jar to 
avoid duplicate class files.We’ve hopefully also prevented future occurrences by forcing all jar tasks in the project to fail by default 
if duplicate classes are added.

Fixes #2452.