Yes that was the question, thank you.
It is correct that for example 1 sub-category with the exact same list of products appears in 2 differents categories (but with a unique URL).
So as you say in the second part of your first paragraph I should merge them or 301 redirect one of the categories to the other.
If I understood your answer right, would it be better to choose 1 category (let's call it category1) as a "father" for this sub-category ?
And maybe create a 301 redirection link for this sub-category that would be present in the second category (category2) ?
Thank you,
Max