This is definitely common, especially if you two are at least somewhat attracted to each other. For instance, I have a best friend who I'd flirt with when we are alone and he'd flirt with me. We also kind of act like a couple too by being open with each other (about problems and stuff), having cute nicknames, and teasing each other. Believe it or not, it never led up to anything romantic (and probably never will since I go to college somewhere far away and he's still in high school) so I believe that our act of flirting is more of an act of endearment rather than being romantically involved. In fact, most flirting doesn't lead to anything romantic and is just an act of endearment or attraction. That's why if you are doing this with your best friend, I don't think it's unusual. Yes, even if both of you are the same gender and/or sex, I am still not surprised. And hey, if it does lead up to something more than that's okay. If not, then flirting is still okay too.