ASP.NET 2.0: Roles.AddUserToRole(...) SqlException

[ijoxley]

I am having trouble adding any user to a role using
Roles.AddUserToRole(username, rolename): whenever I call this method I
receive an SqlException along the lines of:

Violation of UNIQUE KEY constraint 'IX_aspnet_Users_UserName'. Cannot
insert duplicate key in object 'aspnet_Users'.
The statement has been terminated.

This to me seems to indicate that it is trying to add a new record to
the aspnet_Users table - all I am trying to do is modify the roles
assigned to an existing user.

I get this same error when trying to manage user roles using the
ASP.NET Configuration tool in VS 2005.

Has anyone else come across a similar problem?

Thanks

Ian