Slot duplicated when teacher uses "Refresh" button

(from a comment on the plugins database)

We were testing scheduler under Moodle 3.2, and we came across another bug.

The bug will appear when the teacher is trying to give one time slot for a group of students. When the students book this slots, the teacher will get a conflict message telling him to check the ignore conflict button, but eventually he will get 2 slots with the same time and with the same number of students, which means if the teacher is trying to give a slot for 3 students, he will have 2 slots with the same time and 6 students can book those slots.