When you are unhealthy, Duality is probably the best kind of relation to have – technically.

However, as you have alluded to, in reality many people are "hypersensitive" and extremely neurotic when they are unhealthy, so someone who is strong in their weak spots could generally feel like a threat to them, even when it is a Dual. This effect is especially strong if the person has had no to little Duality experience.

I've come across people like that before, and I used to be one of them: someone who strongly needs help from a Dual, but would not be receptive to it.

And that may be strange, considering that one's Dual is supposed to be "perfect", no?

Well, it is not that simple. Duality can often be a bumpy road in the beginning, for both people are so different in certain ways, but they share a synergistic common ground – this fact comes to the surface after extended exposure. A very neurotic and hurt person is unlikely going to reach that stage with their Dual though, they'll withdraw at the faintest idea of possible trouble or difference. Perhaps a healthy Identical would be a better alternative in those particular cases, for they won't appear to be "threatening" or unsettling in any way, and it is said that Identicals can be the best teachers.

Personally, most of my life I used to find my Dual sort of normal, not particularly interesting, too different (lifestyle-wise), etc.
Other types were more interesting in comparison. Regarding my Conflictor, I almost always feel immediately somewhat on the edge with them, or do not want to get to know them.
I suppose growing up with a Conflictor father has given me the ability to spot my Conflictor very quickly, and I usually feel turned off by them. I experience a similar thing with my Supervisor. It's the Fe Role/Te lead combination I've learnt to spot quickly.

To differentiate between a Conflictor or a Dual, you have to get to know them, that's really the only way. They can appear to be very similar from afar. On a closer interaction basis, if you are good at typing you'll figure it out within 10-30 minutes whether they are your Conflictor or not.

To answer the first question directly: No, Duality cannot actually become as bad as a Conflictor relationship, though if you are both very unhealthy and at least one person is being abusive in any shape or form, it could be close to a Conflictor relationship. The focus lies on abusive here, however. I'd assume that over time, if you both love each other and do not abuse each other, you'd be able to become healthier together in a Dual relationship over time. Though there are always outside factors that could make the individuals unhealthy, so it is difficult to say how a particular relationship turns out without knowing significant details.