In this context, duplicates are several references to the same work (article, chapter, book, etc.). These can represent a problem if you cite different references during the writing process. Then references to the same work can be included twice in the literature list, and you will get discrepancies with regard to updates etc.
It is therefore recommended to always check for duplicates in the library when you have imported new references that you are not entirely sure are not already in your library.
When you cite references from different sources, you can easily get references to the same work with small differences in how the information is presented. Some have only initials on authors while others have full first names, some have journals with captions in the title while others have without etc.
Then it is useful to be able to specify which fields to check and possibly how to check.