Keep in mind that a node will not forward a transaction if it has already seen one that conflicts.  

... the _only_ person who can double spend is the owner of the coin (since only he knows the private key). The only time clients see a double spend is if the owner is purposely being malicious. It is therefore reasonable to drop _both_ the original and the second transactions if a double spend is seen within a window of time. Further, the double spend "event" could be bundled (both original transactions included in the bundle as proof of the double spend) into a transaction that locks that coin out for a long time (remember, we have proof of malicious intent so we would be justified in locking out the coin for say 10,000 blocks or more).

After the [15 second] window of time expires the second transaction is simply ignored. At this point we don't want to lock out the coin because we don't want to leave an opening for the original owner to cancel the original transaction. We would need the window to be large enough that we are very confident most clients have seen it yet short enough that the selling merchant can wait without burdening normal customers. 15 seconds for example. So, waiting 30 seconds without seeing the coin lockout transaction would assure the merchant no double spending happened.