Don't get me wrong. I like the crowd funding model, but the real world is not so perfect. We should not say "I won't pay for this, since somebody has already paid for it. " That puts everyone in a bad situation.
My policy: Enjoy something? Contribute back (or pay forward) in some tangible way.
There is also the aspect of risk. Do you pay someone before or after they make a product in an unknown market (E.g. Will this game be successful?). Paying for it rewards them for taking that risk.
Of course, if you commission something beforehand, you take away some of this risk, and the artist should gladly part with some of the rewards.