Duncan struck in the 6-0 win and posted a message to social media.
He stated, "A day I always dreamed of. Making my Liverpool debut.
"A club that I have admired from a young boy and also supported all my life. Also delighted to top it of with a goal and a fantastic team performance. Ps hope my bro Paul Glatzel has a speedy recovery."
Duncan's iconic cousin had a special message in response, as the Rangers boss proudly commented: "Well done kid."