On my installation this is what is happening.
We only allow users to upload to one category called “Records awaiting validation.”
Admin then moves these records to the most apt category e.g. “Ford cars models 1980 to 1985.”
We had a problem with a user who always calls his image the same “scan.jpg.”
The first record was uploaded to the “Records awaiting validation” category and then after admin checked for errors moved it to the “Ford cars models 1980 to 1985” category. The image files were named 61scan.jpg, 61scan-thumb.jpg and 61scan-large.jpg.
A week later the same user uploaded a new record with the same image named “scan.jpg” to the “Records awaiting validation” category. There wasn't a problem with duplicate images as the previous record had been moved, so the files generated were files named 61scan.jpg, 61scan-thumb.jpg and 61scan-large.jpg.
When the new record was moved to the “Ford cars models 1980 to 1985” category (after admin had validated it,) its images over wrote the previous images that were named the same.