I have something along the lines of:

task foo(type: ShadowJar, dependsOn: fooClasses) {
	from sourceSets.foo.output

	configurations = [project.configurations.fooRuntimeClasspath]

	archiveBaseName = 'foo'
	zip64 = true

	mergeServiceFiles()
	mergeGroovyExtensionModules()
	transform(AppendingTransformer) {
		resource = 'META-INF/extensions.idx'
	}

	duplicatesStrategy = DuplicatesStrategy.FAIL
}

There are multiple META-INF/extensions.idx files which, when duplicatesStrategy is NOT DuplicatesStrategy.FAIL, get merged correctly but when it IS DuplicatesStrategy.FAIL - it blows up with org.gradle.api.file.DuplicateFileCopyingException: Encountered duplicate path "META-INF/extensions.idx" during copy operation configured with DuplicatesStrategy.FAIL. Am I doing something wrong or is this a bug? I think that having specified that I want those files appended means that it should not blow up with that error, right?