Well...you already have one, so we can't really tell you anything new about it. I guess the reall questios you should decide by are these:
1. Does this one look to be in better shape than the one you have?
2. Do you think your enjoyment from having another of the same is worth that price (which IMHO is very good).
I myself hit a very similar question about 2 weeks ago (http://www.audiokarma.org/forums/showthread.php?t=211459), with the differences that it was a 500 and not a 600, and that the IA I have is not a 600 supreme but a KA-8150.
In my case I opted to let it go, even though that 500 looked to be in beautiful condition - the reason being that I hardly listen to the 8150 for practicality reasons. In your case I don't know how much you'd be listening to it, but you won't be listening to the both, will 'ya?