Many rewards' terms state that they are limited to one (1) per player.  You may get an error message when trying to purchase the same reward again that states SPErrorTypeID=9300 or "Your purchase limit for this reward has been reached."

Most rewards are limited to "1 per player", per offer period.  This means that you can purchase this reward once during each offer period. When the offer period renews you can purchase it again.  Offer periods are subject to change based on the availability and our negotiations with our partners.  If you attempt to purchase a reward that you have purchased in the past and you get a message that your purchase limit for this reward has been reached, then it means that we are still on the same batch of inventory for this reward.  

We implemented purchase limits for two reasons: to encourage our players to try a wide variety of rewards experiences and to ensure that all players have a fair chance to purchase our most popular rewards.