Synchronise Folder, shows images to import, but when run reports No Photos or Videos to Import

I know this is old but there doesn't seem to be an answer given.  I had the same issue and found an answer on another site and thought I'd update this here:

Click Synchronize Folder and check "Show Import dialog before Importing"
In the import dialog UNcheck "Don't Import" Suspected Duplicates"

The images are indeed duplicated elsewhere in the catalog.  The above will find them and you can complete import.  You can then anually delete if they are duplicates

Select the option to show the import dialog before the actual import starts. I see this too sometimes. I add one new photo to a folder, select 'Synchronize Folder' and initially that dialog says there are two new photos to import. When the import dialog shows, it only shows the one new photo. I have no idea what that other image is (never really bothered to find out), but I do have some ideas: it could be a file that is not a photo or a video, or it could indeed be an image that is already imported elsewhere in the catalog. You can check the latter by unchecking to option to not import suspected duplicates and see if that reveals the photo(s).

That means that the photos have already been imported, albeit to a different location (folder or drive)

Are there new photos that you know of, that they are not yet in the catalog?