Why's everyone saying "duality isn't everything" when I feel like it should be "romantic relationships aren't everything"?
You've met a dual and positively identified them, that's awesome and doesn't happen everyday. She wants to be friends, yay!
I think the sensible thing to do in this situation is be "just" friends, without any expectation of it becoming romantic. If it happens, it'll happen naturally - if not, having a good friend is an end in itself. Duals make great friends, as well as romantic partners... and it helps that people are often friends with their identicals, and their quadra members, which makes them good sources of other betas
Of course it might not be psychologically comfortable for you to stay "just friends" with a dual you're attracted to, in which case try to direct your focus away from her and onto other romantic interests, while remaining friends. Nothing good happens if IEIs try too hard.
Though it's kind of the opposite situation, I friendzoned my SLE bf for years... eventually we started hanging out together heaps and it just happened.