Make withJava() safe to call repeatedlt on same target

libraries/tools/kotlin-gradle-plugin-integration-tests/src/test/kotlin/org/jetbrains/kotlin/gradle/NewMultiplatformIT.kt

@@ -295,7 +295,9 @@ class NewMultiplatformIT : BaseGradleIT() {
 
             gradleBuildScript().modify {
                 if (testJavaSupportInJvmTargets) {
-                    it + "\nkotlin.jvm(\"jvm6\").${KotlinJvmTarget::withJava.name}()"
+                    it + "\nkotlin.jvm(\"jvm6\") { " +
+                            "${KotlinJvmTarget::withJava.name.plus("();").repeat(2)} " + // also check that the function is idempotent
+                            "}"
                 } else {
                     it.replace("presets.jvm", "presets.jvmWithJava").replace("jvm(", "targetFromPreset(presets.jvmWithJava, ")
                 }.plus(

libraries/tools/kotlin-gradle-plugin/src/main/kotlin/org/jetbrains/kotlin/gradle/targets/jvm/KotlinJvmTarget.kt

@@ -31,6 +31,9 @@ class KotlinJvmTarget(
 
     @Suppress("unused") // user DSL
     fun withJava() {
+        if (withJavaEnabled)
+            return
+
         project.multiplatformExtension.targets.find { it is KotlinJvmTarget && it.withJavaEnabled }
             ?.let { existingJavaTarget ->
                 throw InvalidUserCodeException(