Same UID Can't Be Written To Database

I'm having a database insertion problem. I make an entry to my registration form, write it to the database; then the next registration entry displays an error that the same UID can't be written to the database. I don't know where to start tracing this, but here is the stored procedure I use and some of the code that might generate it.

            @UserName nvarchar(128),
            @Email nvarchar(50),
            @FirstName nvarchar(25),
            @LastName nvarchar(50),
            @Teacher nvarchar(25),
            @GradYr int
    DECLARE @UserID uniqueidentifier
    SELECT @UserID = NULL
    SELECT  @UserID = UserId FROM dbo.aspnet_Users WHERE LOWER(@UserName) = LoweredUserName

INSERT INTO [table name]
(UserID,UserName,Email,FirstName,LastName,Teacher,GradYr)
VALUES (@UserID,@UserName,@Email,@FirstName,@LastName,@Teacher,@GradYr)

     Protected Sub cuwCreateUserWizard1_CreatingUser(ByVal sender As Object, ByVal e As System.EventArgs) Handles cuwCreateUserWizard1.CreatedUser
        strEmail = CType(cuwCreateUserWizard1.CreateUserStep.ContentTemplateContainer.FindControl("Email"), TextBox).Text
        strUserName = CType(cuwCreateUserWizard1.CreateUserStep.ContentTemplateContainer.FindControl("UserName"), TextBox).Text.ToLower
        strFirstName = CType(cuwCreateUserWizard1.CreateUserStep.CustomNavigationTemplateContainer.FindControl("txtFirstName"), TextBox).Text
        strLastName = CType(cuwCreateUserWizard1.CreateUserStep.CustomNavigationTemplateContainer.FindControl("txtLastName"), TextBox).Text
        lngGradYr = CType(cuwCreateUserWizard1.CreateUserStep.CustomNavigationTemplateContainer.FindControl("txtGradYr"), TextBox).Text
        strTeacher = CType(cuwCreateUserWizard1.CreateUserStep.CustomNavigationTemplateContainer.FindControl("txtTeacher"), TextBox).Text

    End Sub

    Protected Sub cuwCreateUserWizard1_CreatedUser(ByVal sender As Object, ByVal e As System.EventArgs) Handles cuwCreateUserWizard1.CreatedUser
        Dim cmd As New SqlCommand("sp_CreateUser", con)
        cmd.CommandType = Data.CommandType.StoredProcedure
        cmd.Parameters.AddWithValue("@UserName", strUserName)
        cmd.Parameters.AddWithValue("@Email", strEmail)
        cmd.Parameters.AddWithValue("@FirstName", strFirstName)
        cmd.Parameters.AddWithValue("@LastName", strLastName)
        cmd.Parameters.AddWithValue("@Teacher", strTeacher)
        cmd.Parameters.AddWithValue("@GradYr", lngGradYr)

        Using con
            con.Open()
            cmd.ExecuteScalar()
            con.Close()
        End Using
        cmd.Equals(Nothing)

    End Sub

Please let me know if any other info is needed to help determine what's wrong.