There is certainly enough that feels odd about that scenario to suspect there might be some Face swapping going on...
The only thing I don't get is, if that was Jaqen (or a Faceless Man) posing as Arya, then why did we follow her walking through the streets looking afraid and alone at the end of the episode? Clearly in that moment we are meant to feel the vulnerability of Arya, and it seems a strange choice if that is actually Jaqen who put himself at risk of death or suffering as part of a deliberate plan. But in the scene leading up to her geting stabbed there is definitely enough there to make it plausible that she is an imposter. Particularly the strange mannerisms and speech for Arya. I just saw
this collection of images that sums up all the different circumstantial evidence nicely (not sure if it has been posted here yet).
To be honest I actually noticed the girl with the same outfit as Arya straight away when she walked past, but I never thought that was actually Arya; surely the point of wearing an outfit to try to blend in would be having clothes and hair that other regular people have too? Also, even if the theory that the Arya that was stabbed is an imposter is true, it still wouldn't really make sense for the real Arya to be strolling right past her in the street, since a big part of what makes the imposter theory plausible is that the real Arya may be in hiding.
Anyway, we will definitely find the answer and I think get the conclusion to Arya's Faceless Man training arc in the next episode. Also, supposedly episodes 8, 9 and 10 are among the longest Game Of Thrones episodes we have had (and that's the reason some episodes this season have been on the short side; they wanted to make these episodes longer than usual). I think we are set up for an epic run of episodes to end this season after a couple of quieter episodes of build up since the Night's King attacked Bran's cave.
I loved the Hound's reintroduction and storyline this episode. It felt quite distinct compared to regular Game Of Thrones, and reminded me quite a lot of the TV show LOST (something I've seen other people online say as well). The aesthetic of the septon's camp in the hills is quite reminiscent of something like the Others or Dharma in LOST, and the storyline was quite like something from that show as well, but I think the most striking similarity was that intro: LOST frequently began with a sequence where the viewer wasn't immediately sure where they were or what the significance was, then they revealed the presence of a particular character or object and it became clear. That was exactly how things were done with the Hound, and I think it was done really well.