When you copy a photo, or a series of photos, from one folder to another, it is displayed in duplicate in the destination folder in “photo library” mode while it is present in only one copy in the “Finder”. This happens whether you do “option” + “click-move” on the same disk, or simply “drag and drop” from one volume to another in the “photo library”.
Deleting only one of the two identical thumbnails deletes the photo from the destination folder and triggers the appearance of a "? "In the “Photo Library”
. The only solution to restore the image is to take it out of the trash to the destination folder.