Or you could just sent it back to him.
I don't know, call me old fashioned, but that just seems like... you know... the right thing to do. We all make mistakes. Even if he doesn't realize what he did, you should at least alert him to the fact that he did, indeed, send you an extra product.
Geeze, people. All religious beliefs aside, I would like to think that humanity
still has
some semblance of moral structure left.
EDIT: Then again, if you are without religion, and feel absolutely no conviction in keeping it, then do so. I personally would send it back, but I guess it all comes down to what YOU think.