If Lightroom shows them as greyed out, it means they are already present in Lightroom.
You might want to do a search for one or two such images by their filenames to confirm if they already exist in that catalog.
Follow the below steps :
- Choose All Photographs folder in the Library module
- Enable filter bar by going to View menu > show filter bar (if not already enabled)
- Select Text as the library filter
- Type in the filename in the search bar and check if it shows results.
Thanks Chetna, I've done that and yes the picture appears on the screen but it says "photo is missing"
Ok thanks, I'll have a look at that later