If the same file twice is introduced, whether it should warn the user

[fsy0718]

I have 3 files:

• A define some placeholder,example:

  @define-placeholder test {
     color: red;
  }

• B import A and extend some placeholder, example:

  .test {
    @extend test;
   }

• C import A and B, this file is the master file; when i complete the gulp task, it has two .test

.test{color: red}
.test{color: red}

When I removed the import A from B, It's work right;

[Jonathan Neal]

postcss-partial import is now a layer of sugar atop postcss-import, so duplicate files are now skipped by default.