Course run: Two registrations on the same course

[PanchoutNathan]

Bug Report

Problematic Behavior It is possible to register for two different sessions for the same course

Expected behavior/code It should not be possible to register for two sessions for the same course. An error message should appear for the front, and the back should forbid this possibility

Steps to Reproduce

1. Create a GRADEO with at least one course with multiple sessions (A and B)
2. Open two tabs with this GRADEO syllabus
3. Enroll for session A in the first tab
4. Enroll for the session B in the second tab without refresh