This message appears when you attempt to re-import pictures from a device that you’ve already imported them from (but maybe you didn’t delete them the first time?). The reason is that Windows 10 keeps a record of all the images you’ve imported from your devices using the Photo Import Wizard/tool, presumeably to prevent you from importing a bunch of duplicates. To resolve, we’re going to rename the file where the history is kept, so that Windows does not see it when we attempt to import.
Rename (or delete) this file (to PreviouslyAcquired.db.bak perhaps) and you will be able to re-import.
Credit goes to Steph_Puck on the MS Answers forum.