The site doesn’t seem to catch it when you order an item that you have already purchased. Fortunately I caught this one. It can be a problem because I have been seeing a lot of promotions for “older” products. The example I’m citing is the Speedlight Set Number 1.
If you’re running Windows you can use this for checking for duplicates (it’s free):
It’s not 100% reliable because of occasional inconsistencies between product names in the store and in people’s histories but I believe it will catch at least 95% of the duplicates. Remember to read the Help file.