Issue 27515. Report DUPLICATE_PAR when the same library contains two part directives with the same …

pkg/analyzer/test/generated/compile_time_error_code_test.dart

Index: pkg/analyzer/test/generated/compile_time_error_code_test.dart
diff --git a/pkg/analyzer/test/generated/compile_time_error_code_test.dart b/pkg/analyzer/test/generated/compile_time_error_code_test.dart
index 2e25165ce18875c21673a8b3691f1c73eb227848..927be4948078071571d1e0ff05c55e39e8d0d2f1 100644
--- a/pkg/analyzer/test/generated/compile_time_error_code_test.dart
+++ b/pkg/analyzer/test/generated/compile_time_error_code_test.dart
@@ -1959,6 +1959,18 @@ main() {
     verify([source]);
   }
 
+  void test_duplicatePart() {
+    addNamedSource('/part.dart', 'part of lib;');
+    Source source = addSource(r'''
+library lib;
+part 'part.dart';
+part 'part.dart';
+''');
+    computeLibrarySourceErrors(source);
+    assertErrors(source, [CompileTimeErrorCode.DUPLICATE_PART]);
+    verify([source]);
+  }
+
   void test_exportInternalLibrary() {
     Source source = addSource("export 'dart:_interceptors';");
     computeLibrarySourceErrors(source);