You may have duplicate files.
If so, when you first use the Photoshop Elements Organizer (Windows), Picasa, or iPhoto, or other programs, some of the duplicates may be cataloged.
The above programs won't catalog duplicates that are identical in every way.
But, you can have files with the same file name, that will be cataloged.
That's because if you edit one of the files, or add tags to it, the file is now different from the other version.
Let's say you have DSC_001 in a folder called Jan 2005.
You copied the file into a new folder called Best Ones, and edited the file.
The DSC_001 file that's in the Best Ones folder is now different from the "same" file that's in the Jan 2005 folder.
Both versions of DSC_001 will be cataloged.
Before using a program for organizing your photographs, look for duplicates.
Here are some programs that will help you to find duplicates.
They can be set to search by combinations of file name, file size, date, content, and other parameters.
Test the programs below with a small number of files.
For example, make a copy of a folder with duplicates.
Use this copied folder for testing.
When a program asks, Do You Want to Delete 10,898 files?, you don't want to click Yes unless you're confident!
Find and Delete Duplicate Images John R. Ellis
Duplicate Finder Ashisoft Software
imgSeek also has a search function in which you draw the image that you're looking for.
PhotoSort Free trial
VisiPics 1.10 Free
Duplicate Annihilator iPhoto