I would say go to the file menu and select database (or database management. I don't know exactly what it says) and in the database window create a new database with a different name. when the database is created, remove the old database.
What OS and version of Elements are you using? How did you get into this mess? Where are the original photos stored? Normally, Elements will not import duplicates of the same photo? Do the duplicate photos have exactly the same names or do they, for example, have a suffix attached to them, such as My Photo 100000.jpg, My Photo 100000-1.jpg, My Photo 10000-2? How many prior versions of the program have you had on your computer? Have you imported files from a previous backup?
. . . . or if you don't want to clean up the mess, you can do as edwinb suggests and create a new catalog in the File>Manage Catalogs dialog. But depending on how you have organized your photos, you may lose some of your Albums and Tags and other organizational work. (There are ways to overcome some of those problems too, but we need to know more about how you got into this situation and what you want to salvage.)