Scheduler has, for a long time now, had a feature that prevents "scheduling conflicts" in some situations. In particular, teachers are warned if they are about to create a slot that overlaps with one of their other slots, in the same scheduler or elsewhere. There is no such warning for students however.
Over time, I have received a number of comments regarding this feature; some were confused about how it works, and others asked for it to be changed or expanded.
For the upcoming release of Scheduler 3.1, I have reviewed the "scheduling conflicts" feature, slightly altering how it behaves (with the intent of cleaning it up); and in particular, I have documented the current behavior:
https://docs.moodle.org/31/en/Scheduler:_Conflicts
I'd be happy to receive any comments on whether this feature is useful in the current form or should be changed. It's mostly a trade-off, I think, between being helpful to the users (through stricter consistency checks) and annoying them (by blocking them from legitimate use cases).
Best wishes
Henning