Make Jar build fail on duplicate files

gradle/java.gradle

@@ -30,6 +30,23 @@ if (configurations.find { it.name == 'jmh' }) {
   eclipse.classpath.plusConfigurations += [configurations.jmh]
 }
 
+jar {
+  /*
+  Make Jar build fail on duplicate files
+
+  By default Gradle Jar task can put multiple files with the same name
+  into a Jar. This may lead to confusion. For example if auto-service
+  annotation processing creates files with same name in `scala` and
+  `java` directory this would result in Jar having two files with the
+  same name in it. Which in turn would result in only one of those
+  files being actually considered when that Jar is used leading to very
+  confusing failures.
+
+  Instead we should 'fail early' and avoid building such Jars.
+  */
+  duplicatesStrategy = 'fail'
+}
+
 task packageSources(type: Jar) {
   classifier = 'sources'
   from sourceSets.main.allSource