In lightroom it's possible to ignore photos, that are already in the catalog. Thats fine.
However, sometimes there are situations where imported photos are already processed (e.g. rejected and deleted from the catalog) but without deleting the origin source of the photo, lightroom would re-import these again. Especially with SD-Cards that are not formatted right after the import, its often happening.
For me it would be helpful to have another status "already processed" beside the existing "already in catalogue" and "new". On import i can decide on my own if i want to refuse the files or reimport this again.
Technically i could imagine to use some kind of unique hash which any image should have (?) and save this to a catalog database.