You don't want to import the backups of your photos. You simply copy them, using your operating system, to the folder where Lightroom expects them to be. Lightroom should then recognize them.
If you cant find tehm in Lightroom, you need to try all 4 of these steps:
- In Lightroom, go to the Catalog section on the left and click on All Photographs
- Turn off filters (Ctrl-L once or twice)
- Expand all stacks: Photo->Stacking->Expand All stacks
- Search for at leasst one of the photos by file name, using the Lightroom Filter Bar
Well, what I was trying to do was to import photos from a backup drive to get at least original NEF files (the backup was created at time of first import of files from an SD card to Lightroom). What I expected was that Lightroom will gray out files which are already in the catalog and let me import the missing ones - but it does not seem to recognize them and basically wants me to import also photos which are already visible in the Library
Unless you an conclusively confirm that the files are not in Lightroom (and it sure sounds like they are still in Lightroom), then importing is not a good thing to do. Importing is a very bad thing to do in this situation, and will not solve your problems.
You need to locate the photos in Lightroom via this 4 step procedure.
I copied the files from backup drive and re-synced the folder. The photos appeared in LIghtroom. Then I did what you suggested -went to "All photographs", expanded all stacks... And now the previously missing files appear twice in the Library grid under the same filename.But as soon as I go to the specific folder, they appear just once... So anyhow it seems to me the catalogue got corrupted in some strange way...
Sounds like a user error, re-synching a folder might do this. But there is no way the same file with the same file name can appear twice in a single folder, this is an operating system restriction.
In any event, you might want to go back to the view where the files appear twice, and then clearly the files are no longer missing. Delete one copy from Lightroom (the one you don't want, probably the one with no edits), and work with the other copy (the one you didn't delete) from now on.
That's what I did - I removed the duplicates from the Lightroom catalog in "All photos" view. Of course there were no duplicate files,I know this