The young Reds overcame Manchester City 5-3 on penalties after the two sides could not be separated over 120 tense minutes at City's Academy Stadium.
After the hosts took the lead in first-half stoppage time through Nabil Touaizi, Duncan levelled with a long-range equaliser in the 86th minute to take his tally to 32 goals for the season.
Captain Paul Glatzel then scored the winning spot-kick as Barry Lewtas’ team claimed the club’s first Youth Cup title since 2007.
Post-match, Duncan and Rhys Williams, who was his usual commanding self at centre-back, spoke to Liverpoolfc.