Import doesn't recognize photos already in catalog

Hi everyone. Recently come back from a trip with several thousand photos, I'm having an issue.

I've already imported all of my photos to Lightroom—I think. Upon connecting a CF card back to Lightroom for import, though, many (or all?) photos on the card are not being recognized by Lighroom, even though they are already in the catalog. Yes, the "Don't Import Suspected Duplicates" is checked. Some of the other cards from the trip, when connecting to Lightroom for import, result in a blank screen after the thumbnails flash on the screen, indicating they are recognized duplicates.

This problem worries me because there are well over a thousand photos on the card and I want to be absolutely sure they are all really on my computer and in the Lightroom catalog before I erase the card. Closing the import and searching through the catalog and files on my iMac, they seem to be there, though they are difficult to count and confirm because of several cameras and many folders.

Was there some sort of problem on import I missed? What can I do? Is there a way to tell Lightroom, hey, this photo is in the catalog! Are some photos there but not there somehow? Like, uncounted or won't show in keyword searches or something?

It's probably an unrelated issue, but Lightroom is also having issues with some movie files from the same camera, with the thumbnails not available during import and an error resulting in the files not being imported. I'll ask about that separately unless someone thinks this issue is related.

Any help would be very much apprecaited.