Automatic requires detection should have unique values

[willemdewit]

The automatic detection of which other mixins are being used in a mixin outputs a mixins multiple times when being used multiple times.

[pascalduez]

Shouldn't the fix in #294 be backported to 1.0 ?

[KittyGiraudel]

@FWeinb ?

[FWeinb]

We could do that.