I just wrongly flagged the question Is this correct? $rH=Hr^{-1}$ as a duplicate of the question Show that the set $\{r^{-1} : r\in R \}$ contains exactly one element out of each right coset of $H$.. While the questions are related, they are not the same. Unfortunately, I only realized that when the first answers showed up.
It seems that there is no way for me to revoke the flag. I've tried to raise a second flag explaining my error, but it turned out that more than one flag on the same question is not allowed. Is there any way to tell the moderators they should ignore that flag (and thus saving them time)? What is the correct behavior in this situation?