I tend to agree with @ashlesha here. Maybe it's a sensor thing, or maybe it's an ethical thing (if we're looking at it from a socionics standpoint) but evaluating people based on how they're going to contribute to my life in the long term or based on their assets is not something that I'd do consciously. If I were in a relationship with someone and having problems, I wouldn't be thinking, oh crap, I could have found a better person, I should leave this person immediately and go look for that one (like, what you'd do with an awful lunch or something). I'd try to fix the problem first. But I guess that's what you mean by "taking a risk"; if you break up with someone 85% good for you in search of the Perfect One, you risk never finding them and losing those 85% too. I don't think relationships can or should be classified in that way, though, because dual or non-dual, a relationship will fail if you don't put effort into it. AND people make bad choices, as ashlesha said.
With that being said, I think duality isn't really applicable when we're younger. Putting the risk think into perspective, you take a risk every time you choose to enter a relationship. And you're going to learn something about yourself whether you break up or you stay in it until death do you part. The more relationships you have -- or the longer you are in one relationship -- you should be better at recognizing what you like and don't like in people. And also become better at accepting criticism, which comes with age. So if you're looking at duality as the only viable choice for a relationship... you're barring yourself from learning a lot about people -- yourself included.