The ending is just perfect and I keep thinking about the commentary track where Russell and Carpenter discuss the nature of the thing. If it perfectly copies you, then do you necessarily know that you are The Thing until you are tested or seriously wounded? Obviously after that the defense mechanism kicks in and the real deal appears, but in the beginning is it more like an infection inside you or should I say your copy? Maybe either MacReady or Childs (or both of them) were already copies at the end and they just didn't know it.
Awesome movie and overall it follows the original story more closely than the 1951 version.
I like to think that it's more like an actor taking on a role. As much as they would get into the emotion and mood of the role, at heart they know they're an actor. The Thing would always know it's itself no matter how perfect it duplicates the victim.
Either way, great movie.