Existing identical font-feature-settings gets duplicated

[ChaosExAnima]

Currently, given CSS as follows:

selector {
  font-feature-settings: normal;
  font-variant: normal;
}

The code produces the following after processing:

selector {
  font-feature-settings: normal, normal;
  font-variant: normal;
}

This code makes sure that if there's an existing font-feature-settings value, the new value being appended doesn't match what currently exists.

[MoOx]

Thanks!
Released as 2.0.1.