Since you have the box <Don't import suspected duplicates> checked, the only reason why "duplicates" are imported is that they have different file names than the images already imported.
As you probably know, Lr cannot recognize the content of images. For Lr two images are identical when their file names are identical; and images are not identical when their file names are different.
The file name includes the extension that designates the file format, such as "jpg", "tif", "psd". So, two image files with the same name are not identical when the file format is different.
Also, as I have noticed Lr "sees" underscore "_" and hyphen "-" as different things, and thus treats two images files that differ only in underscore vs. hyphen as different files.
Some people have selected - in their OS - that the extensions for "known file types" are not shown ("Hide extensions for known file types"). If you have done this change it back so that the extensions are always shown.
In Windows this is done in >Control Panel >Appearance and Personalization >Folder Options >View tab.