duplicate_part

The library already contains a part with the URI '{0}'.

The analyzer produces this diagnostic when a single file is referenced in multiple part directives.

Given a file part.dart containing

part of 'test.dart';

The following code produces this diagnostic because the file part.dart is included multiple times:

part 'part.dart';
part 'part.dart';

Remove all except the first of the duplicated part directives:

part 'part.dart';