I don't know, I think I would be kind of bothered by that. If my boyfriend's last girlfriend was someone who looked and talked like me, had the same attitude as I did, and worked the same job, it would be weird. Especially having had him tell me already that at the end of the relationship he had decided that his ex was a crazy bitch. For someone who is working on trust and opening up and needing to take things slow, knowing how much anger he had towards his ex and how similar you are to her is a little scary.
I have actually had this happen to me. A guy I was dating had previously dated a girl that I knew that looked EXACTLY like me--like, to the point that when she started going to my school people started asking me why I never mentioned I had a little sister (I didn't). Only in that situation (and possibly in this one?) he had a thing for me long before she was around, and she was, in some ways, him settling for an alternate me. Which is pretty creepy in hindsight, but I didn't know that when we started dating, and was actually resistant to dating him because I thought the reverse was true--that he was substituting me because he couldn't have her. I hope that turns out NOT to be Faye's situation, because my ex turned out to be a total creepface in a myriad of awful ways, but I'm just saying, when it happens, it does feel a little weird and puts you on the defensive about the authenticity of someone's feelings. Especially when you don't have any contrasting cases of other people they've dated to reduce the sketch factor.
But the more we talk about this, the more I feel like Faye might get a little taste of Dora's anxiety right after she started dating Marten--and by putting it in that context, it might actually make it easier for her to be like, "this is actually not a big deal and I'm not going to let it get me down," because she's seen where that road goes.