Events and appointments can be marked Busy or Free. When you save a Busy event or appointment that overlaps another Busy event or appointment, Organise You warns you before saving.
The warning never blocks you permanently. Choose Save anyway if the overlap is intentional, or change the time. Items marked Free do not trigger overlap warnings.
You can also set Attendance to In person, Online, Phone or Not specified. If one of the overlapping items is online or by phone, Organise You notes that the overlap may be intentional, but still warns you because you may still need your attention in two places at once.