What are 'dup0' files in the photos folder?

When I wanted to copy some photos and videos from my Samsung Galaxy S to my PC, I noticed that inside the photos folder (DCIM\Camera), there are a lot of files with the extension dup0. They have the same filename as the photos/movies (e.g., img1.jpg and img1.dup0), but not for every photo does a dup0 file exist. The dup0 files are quite small for photos (e.g., photos are about 1.5 MB, dup0 files about 50 kB), but for movies, they can be quite large (e.g., 16 MB for an 18 MB movie). What are these files for? Can they be deleted to free some space?