If Marion's fiance Peter is the confident, well put together, organized, perfectionist - than Duncan is his opposite.
I think what you have so far is a pretty accuate description of him - but let's analyze. Here is a woman who does not exactly know what she wants, and is drawn to two completely opposite men. Both men represent the two sides of what most women want in companionship. On one hand, Peter represents the confidence, stability, and consistency that women look for when thinking about a worry-free future. Duncan on the other hand, represents spontanaiety, inconsistency, and that laid-back approach to life that says, "I take things as they come."
What Marion really needs is a balance between the two. As in most cases however, Peter is not enough. In fact, he is a bit shallow - and Marion begins to not only realize this, but that Peter wants her to evolve into perfection for him. Duncan on the other hand, is not enough either. He is enticing and fun. In fact, he helps Marion discover who she really is because at first, she wishes to impress him (perhaps be more "real" like him) but when she realizes he is just as shallow as Peter, she starts to understand what she really wants and needs, based on what these men aren't.