Another Roles Tutorial problem: User is already in role , or User is already not in role

I'm having an issue with the ASP.NET Roles Tutorial, using VS2010 B2, and the language is C#.   Even using the sample code, exactly as downloaded, the checkbox_changed event for the Roles repeater seems to be raised incorrectly.   If I go to the RolesAndUsers.aspx page and add a checkmark to the currently selected user's list or roles, it correctly raises the CheckBoxChanged event.   Following along with the debugger, I see that we step into the event handler, which correctly sees that the CheckBox is checked, and the user is added to the role I have checked.   But, if I decide to reload the page, then the event is raised again, this time throwing an exception because the user already belongs to the role I am attempting to assign him to.   
To watch this happen in reverse, I only need to clear the checkbox for one of the user's existing roles.   First pass through the event handler, it works.   Second pass through, it fails b ...