I don't know of free software to do this.
You can find dups using XP to search all your photos and then by playing with the different view settings you can recognized the dups. Dups will have the exact same file size, so listing by size is a possibility. The other is listing by date photo taken. Depending upon the number of photo files you have this can be easy or tedious.
Here is how to do it.
Click on Start, click on Search.
Click on Picture Music or Video
Click on Pictures and Photos
In the textbox "All or ..." type exactly this:
*.jpg
and click Search
This will return all the photo files (.jpg) on your hard drive
Then look at the menu on the left and click on Sort results by category and click on Size
Look at the menu on the left again and click on View results differently and click on Details.
Enlarge this listing window full screen so you can see everything.
You will see the columns headers: Name, Folders, Size. Right click on one of the column header name and a list will open. Click on More
A window will open, scroll down and check the box "date picture taken" and this will be added as a new column.
You now have the listing, click on Size to list by size instead of by Name. The list will start with the smallest file, these are unlikely to be your photos. Click on size again to list from largest to smallest. You can then inspect for dups as you go down the list and check the name of the files of two exact same file size and just delete the one you don't want.
If you click on the column "date picture taken" the first files listed will be files with no such data. Click on "date picture taken" again and you will now see the files in descending order.
This is effective only if you change the XP default from listing date in DD/MM/YYYY format to YYYY/MM/DD format so the listing is displayed in an easier way to recognize the dups. Obviously this only works if you have a digital camera saving such data, most cameras do and XP will read the info imbedded in the file.
Ist there a free program available to find dups?