There are a lot of modules out there focused on upselling, but sometimes you need to downsell. If you run a shop where customers frequently buy large numbers of low-priced items (digital scrapbooking, MP3s, etc) then you have probably dealt with customers buying items they have already purchased. After they make this mistake, you become responsible for issuing a refund or credit for the twice-bought items. It is time-consuming and often a frustrating hassle.
The Already Purchased Alert module shows a display on the product page or the checkout/cart page to alert customers that they are about to make a repeat purchase. This greatly reduces the number of accidental repeat purchases, and eliminates the need for time-consuming refunds for you and the customer.