Execution failed for task mockableAndroidJar, mockable-android- . jar already exists

Simply cleanup the existing jar before running the task again. Running cleandoes work, but it's a little intense in my opinion. Instead, make the android-generated task depend on a very specific cleanup task:

task cleanAndroidMock(type: Delete) {
    description = 'Deletes the mockable Android jar'

    delete fileTree("${project.buildDir}/generated") {
        include 'mockable-android*.jar'
    }
}

project.afterEvaluate {
    tasks['mockableAndroidJar'].dependsOn cleanAndroidMock
}