Picasa has a duplicate file finder but it's hard to tell what you are looking at: Run Picasa; click
Show Duplicate Files. It works but be careful about what you are deleting.
Another option is the free "Fast Duplicate File Finder":
Fast Duplicate File Finder
What I liked about this is it allows to you preview the picture before deciding you don't need the duplicate.
That works fine as long as the files are identical, but if you are looking for similar photos (e.g. same photo but resized, rotated, or manipulated), you may want to look into their upgraded version:
Visual Similarity Duplicate Images Finder
I haven't used this program but I used a very similar program (no longer made) and I found it very useful. They have a free download so it might be worth checking out.