"A Duncan" is the person that you fall in love with,early on in life.Your first love. Every new partner is measured by Duncan's standards.
Obviously, one rarely ends up with one's Duncan as a life partner. However until one is thoroughly over "Duncan" one cannot move on.
Even though "Duncan" is a male name, your "Duncan" can be whichever sex your first love belonged to.
Sometimes, it can take a great many years to realise that someone was your Duncan, but once the realisation has been made, one can start to come to terms with the reality that they probably weren't all that special, and most likely are nothing special at all *now*
In other circumstances, the Duncan relationship may not have worked out becuase your Duncan was a closet homosexual, yet was completely perfect for you in every other way.
"Remember X from <wherever>? He was my Duncan"
"Oh, Cyril from school was amazing, he was my Duncan"