In summary: for the time being, just ignore these suggestions, they're harmless.
Upload image file with the file dialog or by dragging and dropping images onto the dashed region
https://forums.zotero.org/discussion/103977/add-ability-to-mark-multiple-items-as-not-duplicate-rather-than-merge-them
https://forums.zotero.org/discussion/123342/erroneous-duplicate-items
Quoting aborel in one of the answers: