How old is your Photopost and version? As if its very old this might be the issue cause I think older versions allowed files with the same name if they where not in the same cat. Thus if you moved them to another cat and they had the same name this could happen.
There should never be an issue with files overwriting one another in never versions of the software since we employ a file rename structure in place when photo is uploaded we check all photos to make sure the names are not the same.
Not much I can suggest in this case as you can not get that photo back without backups but Roxy can edit the photo and upload a replacement photo.