How to stop duplicate photos

Simply delete them, if you have them all in your Pictures folder, choose sort order by file size, that should bring them all together or fairly close to make it easier if they have different names.
As to how to stop duplicates, it depends how you get them, same name to same folder will show a duplicate error when copying.
Basicly it depends on how you organise your photos and where you get them from. Digital cameras generate a unique file name for each new photo.