@AtticRattata asks
bought a card this weekend, got home, already had it. That's twice now. Please tell me there is a system for people like me lol
Jeff answers
I keep one list I can check before buying, and I update it when a card comes in or leaves. Include the version, not just the Pokémon's name, because similar copies are easy to mix up. Before a shop visit, take a quick look at your recent additions. The list doesn't need to be beautiful; it just needs to be current enough to save you a duplicate.
— Jeff, infinitepulls.com