Not all photos from import from another catalog were imported

[serial number]

Hi,

When I was on a trip I used Lightroom Classic on my lap top. When Home I exported as catalog then imported from another catalog. This has worked on several days but the last day I did of 3101 photos only 2878 were imported. The folder containing the photos in the export contains all 3101. It seems to think the remaining photos are duplicates. I have not been able to find duplicate photos in the catalog.

Please help.

Thanks

Bill