I was suggested to translate article which I had already in progress. See: https://imgur.com/a/Vqoqd for screenshots.
Currently users cannot translate an article which is being translated by another user (T86151).
Thus, it does not make much sense to suggest an article for translation if another user already blocked it.
Even if users are somehow able to translate the same topics, we probably don't want to encourage it if there are alternative choices.
This filtering is currently done for the local recommendation algorithm (the fallback one), but not for the one providing personalised recommendations. That is specially problematic for new users since their lack of previous translations (used as a seed) makes them more likely to be exposed to the same set of suggestions (randomising the suggestion order can alleviate a bit this problem too).