Need help with Application error with AddUsersToRole method

I have a really strange issue with my admin console when attempting to add a user to a specified role.  The issue is strange in the fact that I get the error that the user already exists in the role when I try to add them.  The user is being added the to role just fine but it raises the error that the user(s) already exists.  Would appreiciate help with this as it is got me stumped.  I have included my code behind function below.  Thanks in advance.
Protected Sub AddUsers_OnClick(ByVal sender As Object, ByVal e As System.EventArgs) Handles btnAddUsersToRole.Click ' A role must be selected' If RolesListBox.SelectedItem Is Nothing Then Msg.Text = "Please select a role." Exit Sub End If ' At least one user must be selected' If UsersListBox.SelectedItem Is Nothing Then Msg.Text = "Please select one or more users." Exit Sub ...