The reason for the double check of the boarding pass is this.
Some boarding passes are marked with an "SSSSS" to indicate that the computer has flagged the passenger as a security selectee. These folks will get the hand wanding and hand inspection of their bags regardless of the results of the walk-thru detector and Xray bag screening.
The first checker will direct selectees to the appropriate line but they don't escort them so it's possible that they might switch lines. The second check is to verify that the person going through is not a selectee who switches lines. Also, the line for selectees will also process non-selectees so they have to check to see if you're a selectee or not.