Support diamond inheritance of sealed interfaces in SealedClassSerializer (#1958)

Fixes #1937

Author: sandwwraith

core/commonMain/src/kotlinx/serialization/SealedSerializer.kt

@@ -105,7 +105,7 @@ public class SealedClassSerializer<T : Any>(
             element("type", String.serializer().descriptor)
             val elementDescriptor =
                 buildSerialDescriptor("kotlinx.serialization.Sealed<${baseClass.simpleName}>", SerialKind.CONTEXTUAL) {
-                    subclassSerializers.forEach {
+                    subclassSerializers.distinct().forEach {
                         val d = it.descriptor
                         element(d.serialName, d)
                     }

formats/json/commonTest/src/kotlinx/serialization/features/sealed/SealedDiamondTest.kt

@@ -0,0 +1,40 @@
+package kotlinx.serialization.features.sealed
+
+import kotlinx.serialization.*
+import kotlinx.serialization.descriptors.*
+import kotlinx.serialization.json.*
+import kotlin.test.*
+
+class SealedDiamondTest : JsonTestBase() {
+
+    @Serializable
+    sealed interface A {}
+
+    @Serializable
+    sealed interface B : A {}
+
+    @Serializable
+    sealed interface C : A {}
+
+    @Serializable
+    @SerialName("X")
+    data class X(val i: Int) : B, C
+
+    @Test
+    fun testMultipleSuperSealedInterfacesDescriptor() {
+        val subclasses = A.serializer().descriptor.getElementDescriptor(1).elementDescriptors.map { it.serialName }
+        assertEquals(listOf("X"), subclasses)
+    }
+
+    @Test
+    fun testMultipleSuperSealedInterfaces() {
+        @Serializable
+        data class Carrier(val a: A, val b: B, val c: C)
+        assertJsonFormAndRestored(
+            Carrier.serializer(),
+            Carrier(X(1), X(2), X(3)),
+            """{"a":{"type":"X","i":1},"b":{"type":"X","i":2},"c":{"type":"X","i":3}}"""
+        )
+    }
+
+}