Why are multiple files uploaded when more than one category?
 

I've noticed that if a member uploads a photo and chooses to add it to a category other than just their member gallery then multiple image files are uploaded and multiple thumbnails appear. What is the best way to upload photos so that only one image file of a photo is uploaded but where it can be assigned to other categories besides a members gallery?

Multiple files are never uploaded. If you have in upload options that they can upload to optional categories set this to OFF

Also if you dont want the members gallery to show a members images elsewhere then you can turn off the option that members gallery shows all a users photos in index options