I'm creating a duplicate or back-up on another external drive. I can see all the images in the import preview darkened to not import duplicates and it will not import any however when I look at the library view there are substantially fewer images and I can't see them. I can look at the drive I copied from and see them all. This new drive is showing 23,877 and the original drive is showing 248,704. I can look at the size and count on the drives and they match.
If you are really, Actually, creating a Duplicate set of image FILES on another drive, Internal or External, as a Backup of all your photographs that BACKUP set of FILES should NOT be imported into the LrC Catalog file.
So I suggest you stop trying to do that as it is not how LrC is designed to work.