Hmm, you are working with IrfanView, then ...
within Irfanview open the dialog for searching files by pressing [Ctrl+F].
Enter a fitting file search mask, for example *_1.jpg
Enter a root folder.
Set the option to search in the foldertree.
Start the search.
Display the result as "Thumbnails".
This works only if the files in one folder do not have embedded art with different properties.
Otherwise, one picture file per version will be created (and numbered).
So it is worthwhile to check the folders where there are duplicate files although they should not. The duplication means that something is different.