Although most of the content in my Library is sourced through Amazon and appears normally in my Library I do have a number of reference publications which come from a 3rd party as MOBI files. These are then sent to my Kindle email account for downloading.
These files appear as DOCs rather than BOOKS which is not a problem - however two copies often appear in my Library - with identical titles and details.
Although both of the copies seem to be downloadable and readable, having the duplicate copies in the DOCS section of the Library is annoying. It also causes issues with the whipsersynch and collections features.
This only occurs on my Kindle Oasis (9th Gen), The Kindle app on my iPad, iPhone and Mac along with the "Manage your Content" section of the Amazon website all correctly display a single copy.
one way you can see two copies on a Kindle is to see one in a Collection and one not in a Collection. If you sort by Collections and not by Recent or Title or Author, you'll only see one. You'll also see one under All and one under Downloaded. These are the same file and if you remove one from your device, they will both disappear.