> He understands that he can have two same meetings without a problem
No, that's not quite correct. In fact, Scheduler will try to prevent such "conflicts" in the calendar. What happens (or should happen) if you create the second slot series, and set "Force when overlap" to "Yes", is that the first slot series gets deleted.
It seems however, by your report, that there's some problem in the deletion process.
Have you tried this in a newer version of Scheduler? Both Moodle 2.7.9 and Scheduler 2.7.2 are now out of maintenance.