Both sides had opportunities to win the U18 Premier League encounter but it was the young Reds who finally found a breakthrough in the precursor to the first-team showdown between the clubs on Sunday.
Duncan produced the decisive moment for Barry Lewtas’ team with a clinical finish that increased their winning run in domestic football to six games.
Liverpool made a bright start in Kirkby and their committed pressing high up the pitch ruffled City as they attempted to move the ball out from the back.
The first sight of goal was forged by the visitors 11 minutes in, however.