Given the likelyhood of finding humans on an extrasolar planet (lost/forgotten colonies and transplanted humans, such as in the Stargate universe don't count) and the vastness of the universe, something that looks exactly like us on the outside could exist somewhere, maybe not in our galaxy. Even THEN, the odds are incredibly low
I don't mean just humanoid to the point of having the same or similar body structure, two arms, two legs, a head and walking upright, I mean exactly like us, like the Star Trek aliens minus the prothsetics and makeup. Having the exact same organs in the same place though, those odds are so incredibly low that it's pratically zero that they could evolve naturally. Not having the same internal organs ups the odds slightly to where it's probable, but remote.
However, when we are talking about humanoids, four limbs, a body and a head, those are definetly possible. Even if they evolved using something other than DNA or are based on something other than carbon, evolution still folows basic rules and something that works well, will get reused over and over again in evolution.