After this user is renamed himself with English, it still impossible to find and add him to the group.
You can add the same user into one group multiple times via the admin panel
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Does it annually do anything? Does the user show up multiple times in the group list or anything?
It doesn't get saved which is good. So only the admin panel has this "bug", where you can add the same user multiple times, but it gets removed once you save the group and reload your tab. So nothing critical.