[Mentoring/Queue] Prevent mentor from starting the same mentoring session twice

Now that it's possible for a student to request mentoring a second time for the same exercise, there should be some sort of mechanism that prevents the same mentor from picking up the same mentoring session again, or at least warns them.

The solution depends on what motives do we expect the student to have for requesting mentoring again. Does requesting mentoring again mean that they were unhappy with the first mentoring? Or are they simply curious about what somebody else might say?

I am assuming that if the student or the mentor wants to continue the initial mentoring session, they can add comments to it anyway, so there is no technical reason for the same mentor to pick up the new mentoring request for the same exercise again.