[Gradle] ObservableSet: Only add listeners when a new object is indeed added

^KT-60937 Verification Pending

libraries/tools/kotlin-gradle-plugin/src/common/kotlin/org/jetbrains/kotlin/gradle/utils/ObservableSet.kt

@@ -40,10 +40,12 @@ internal class MutableObservableSetImpl<T>(vararg elements: T) : MutableObservab
     }
 
     override fun add(element: T): Boolean {
-        return underlying.add(element).also {
+        val added = underlying.add(element)
+        if (added) {
             whenObjectAddedActions.toTypedArray().forEach { action -> action(element) }
             forAllActions.toTypedArray().forEach { action -> action(element) }
         }
+        return added
     }
 
     override fun isEmpty(): Boolean {

libraries/tools/kotlin-gradle-plugin/src/functionalTest/kotlin/org/jetbrains/kotlin/gradle/unitTests/ObservableSetTest.kt

@@ -39,6 +39,26 @@ class ObservableSetTest {
         assertEquals(listOf(1, 2, 3, 4), testListener2.invocations)
     }
 
+    @Test
+    fun `test - adding already existing elements`() {
+        val set = MutableObservableSetImpl(1, 2, 3)
+        val forAllListener = TestListener()
+        val whenObjectAddedListener = TestListener()
+        set.forAll(forAllListener)
+        set.whenObjectAdded(whenObjectAddedListener)
+
+        assertEquals(listOf(1, 2, 3), forAllListener.invocations)
+        assertEquals(listOf(), whenObjectAddedListener.invocations)
+
+        /* Adding already existing elements */
+        set.add(1)
+        set.addAll(listOf(1, 2))
+
+        /* No further invocations */
+        assertEquals(listOf(1, 2, 3), forAllListener.invocations)
+        assertEquals(listOf(), whenObjectAddedListener.invocations)
+    }
+
     @Test
     fun `test - whenObjectAdded`() {
         val set = MutableObservableSetImpl(1)