Add validator for unique serial names

plugins/kotlin-serialization/kotlin-serialization-compiler/src/org/jetbrains/kotlinx/serialization/compiler/resolve/SerializableProperties.kt

@@ -51,6 +51,14 @@ class SerializableProperties(private val serializableClass: ClassDescriptor, val
                 else
                     SerializableProperties(supers, bindingContext).serializableProperties + first + second
             }
+            .also(::validateUniqueSerialNames)
+
+    private fun validateUniqueSerialNames(props: List<SerializableProperty>) {
+        val namesSet = mutableSetOf<String>()
+        props.forEach {
+            if (!namesSet.add(it.name)) throw IllegalStateException("$serializableClass has duplicate serial name of property ${it.name}, either in it or its parents.")
+        }
+    }
 
 
     private fun isPropSerializable(it: PropertyDescriptor) =