Most stores will accept a return for any reason within a short period (usually 7 days to a month). However, some will charge you a restocking fee. Usually 10%-15%.
If the other laptop is *exactly* the same, you could simply buy it from the other store, then give it to the first store unopened for a refund. Usually stores will waive the restocking fee if the item is unopened. i.e. You keep the current laptop, but take the laptop you bought at the second store, and "return" it to the first store for a refund. Say you got it as a gift, but it turned out the person already got a laptop without you knowing it, so you're returning it unopened.
You can also try bringing the ad for the lower price ad to the store, and complain...