Yes, we can. Even identical twins will have different mannerisms. They won't talk exactly the same, nor will they react the same ways to various interactions. They'll hold themselves differently, when standing/sitting; they won't necessarily like the same foods, etc.
Once you know somebody pretty well, that kind of stuff won't fool you.
Yes. Even if they dressed and looked identical, I'd think that, after a few dates, we would have talked enough, and been comfortable enough around each other, that small changes in chemistry between us, or a back reference to what we did before being something she doesn't remember (or remember properly) would tip me off that I was being pranked.