Can mixin handle duplicated extending correctly?

[LongTengDao]

mixin X {}
mixin Y extends X {}
mixin Z extends X {}
class A with X, X, Y, Z {}

It could actually happen for multi using of different libs, they could have shared mixin, but only meant including that, not run twice or more times...

[Atry]

Because mixins are just functions, the function implementation could decide if it should or not skip the transformation when it detected the mixin had been applied.