At the very beginning of Portal, GLaDOS greets the player with these words:
Hello, and, again, welcome to the Aperture Science Computer-Aided Enrichment Center.
Speculation, but it seems like it would be safe to assume there has been interaction with GLaDOS or a human before the game begins, and there would have been initial greetings then. You start out in the relaxation chamber; you had to get into there somehow, and I doubt that you could wander into there with no interaction whatsoever.
"Hello and again welcome to the Aperture Science Computer-Aided Enrichment Center. We hope your brief detention in the relaxation vault has been a pleasant one."
I have a theory!
Well, as it turns out in Portal 2, Chell was brought into Aperture Science at the "Bring your daughter to work day", which, unfortunately, fell together with the "Switch on GLaDOS" and the "GLaDOS kills everyone inside Aperture Science"-Day. If we assume correctly, Chell was in stasis of some kind which still allowed her to grow (or in a growing chamber of some sort) and was awoken by GLaDOS to begin testing. She could have been "stored" in another complex or part of the complex (which is vast as seen in Portal 2), and so the greeting seems to be correct.
I think the intent was just to immediately establish GLaDOS's strangeness; people from Valve claim to have never heard of the "repeated cloning" theory, which is the obvious explanation that would make it actually mean something.