If I accidentally upload the same photo twice, will there be a duplicate in my album?
No. If it is truly a pixel-to-pixel duplicate, dotphoto will only upload the photo once -- even though the same photo has different names.
If you change the image by even one pixel, it becomes a different photo. This can be helpful if, for instance, you want to repeat a photo in a slideshow. If you want an identical photo to appear twice in an album, you could increase the size by 1%, and upload the photo again.
If you want the photo to appear in multiple albums, you can use the Organize | Copy command to place the photo in multiple albums. You cannot copy the same photo to the same album.