"IMO, you should should avoid importing (platform themes) manually, and instead use the mechanisms of the platform. "

The platform doesn't allow to import multiple themes, just descendant.

Can you please clarify what are you trying to achieve?

I don't want to see the same class definition twice.

Filipe, this is not what you're trying to achieve, but the consequence/side effect of that.

In order for anyone to help you (e.g. me, community, OS R&D?), please explain what are you trying to achieve (as a whole) - the scenario that lead to this "finding".

BTW, regarding how browsers work: the classes don't get classes duplicated - the browser computes them into the CSSOM

Ruben i think you are not understanding my issue. If you want you can replicate by doing the steps I described above.

Hello Filipe. 

Right now it seems that the issue is understanding what your issue is - pun intended. My suggestion, in order to speed up any help that anyone might be able to give you, would be to having you either showcasing, through screenshots, what you're facing (and your reasoning for the issue) or, better yet, to create a simple PoC in a module (or app) that showcases what you are trying to achieve - and that replicates your issue.

Cheers!