Fix assertion in light tree diagnostic reporter

happened then two diagnostics are reported on the same element.
#KT-55924 fixed

compiler/tests-gen/org/jetbrains/kotlin/cli/CliTestGenerated.java

@@ -1077,6 +1077,11 @@ public class CliTestGenerated extends AbstractCliTest {
             runTest("compiler/testData/cli/jvm/syntheticAccessorSignatureClash.args");
         }
 
+        @TestMetadata("twoDiagnosticsOnSingleElement.args")
+        public void testTwoDiagnosticsOnSingleElement() throws Exception {
+            runTest("compiler/testData/cli/jvm/twoDiagnosticsOnSingleElement.args");
+        }
+
         @TestMetadata("unknownExtraFlags.args")
         public void testUnknownExtraFlags() throws Exception {
             runTest("compiler/testData/cli/jvm/unknownExtraFlags.args");