Definately not a good idea to forfeit one sector that is part of your ticket by buying another ticket just to get on the earlier flight. The forfeiting will most likely result in the cancellation of your remaining sectors. Only time forfeiting a sector works to your advantage is if you live in city B going to city C where you originate from city A connecting through city B to get to city C to take advantage of a cheaper fare ex-city A where when coming back you fly from C-B, get off at B (where you live) and forfeit the B-A sector which is the final sector of your ticket.