Darn that _is_ a real dilemma... I would have probably returned the second N900 to them too (thinking they'd notice it anyway) but I would probably keep thinking "what if I didn't return it...". Then again, one N900 is already a lot of power, and I wouldn't constantly use 2 of them (it would be nice to have a second just to test things with, but hey).
I do think you did the "right" thing here, what if someone else was actually waiting on an N900 that never came because it was erroneously delivered to your door? I wouldn't want to be that guy...