My duplicates all seem to be "reposts".
When I add it is using a spreadsheet which automatically detects duplicates and adds an extra code to say it is #2, #3 or whatever. The recent notifications of duplicates have all been long established items, reposted, but never added as a new item. Also, they crop up shortly after the item has been reposted.
I know of no way of preventing the repost process from creating a duplicate item by reposting the same item twice.