You'd have to post the relevant code - assuming there is any. Likely more than one event is involved.
Or step through it line by line and see if you can spot the problem. Often it's because you've made some modification to a bound field then made a change elsewhere yet the first change hasn't been saved. To Access it looks like 2 users are changing data at the same time (it can't tell that it's the same person). The record level locking property can also come into play here. The 2 changes might be on separate records, but if record level locking is set to be a large set of records, it's not that the same record is being edited, but the same page (or section) of records.
In summary, try stepping through your code with the above in mind.
The more we hear silence, the more we begin to think about our value in this universe.
Paraphrase of Professor Brian Cox.