I think the exact same question asked in both languages would be a duplicate.
Of course that can lead to the problem that the person asking the second question might not be capable enough in the other language to get the full benefit of the first lot of questions and answers.
But there is a solution. Questions can also be merged by moderators. In the case of a bilingual site like ours it's reasonable to expect that such merging would result in a nice selection of question and answer in both languages.
Naturally that would make merging questions more work than on a single language site.
But we could help deal with that by asking for translations of individual posts in either comments or in meta questions if the comments are already so full that a new one might be too easily missed.
And of course anybody should always feel free to add translated equivalents to any post. I've already done it a few times but it might only work well on rather short posts...