The dotphoto cart does not allow duplicates, which prevents customers from inadvertently ordering more than one of the same photo. (When you want more than one of the same  photo, you can set the quantity to 2 or more.)

With more complex projects like a card, dotphoto looks at the first photo to determine whether the project is unique. Photos that are different by even one pixel are considered unique, so you can use virtually the same image on the same product by cropping it slightly differently and saving it as a new photo.

How to crop a photo