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I think they know that you let the person in, and they didn't come in themselves. Also, I think the barks are meant to do two things, one, intimidate the 'intruder', and two, alert you to the fact someone is there. If you don't react negatively, I think they pick up on that.
Some dogs are just super friendly, though. My dog loves people way too much to be a guard dog. He may look tough, but he just want to have fun play with everyone he sees.
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