cant insert into

Please help. I am new to web development and currently working on a razor web pages site. I have the name of the StarterSite.sdf changed to YourBookBroker.sdf along with adding the changes to the web.config, _AppStart, Register.cshtml, and RegisterService.cshtml for calling the database.
I have been unsuccessful trying to add data to a username column in the userprofile table so i made a "SiteUser" table with UserID, UserName, Password, FirstName, LastName, Address, City, State, PostalCode - columns. I added an additional insert statement below the insert for the userprofile insert. I also changed the form and added a username field. But I am getting "A duplicate value cannot be inserted into a unique index. [ Table name = SiteUsers,Constraint name = UQ__SiteUsers__00000000000000B9 ]" error. I'm not sure why this is happing. It looks like I am doing the exact same process as the insert statement for the userprofile.
This error co ...