Cryptomnesia

Cryptomnesia is a memory trick where you forget that you already know something. It's like you think you're learning something new when really you already knew it. It's like when you hear a song on the radio, and it feels like you're hearing it for the first time, even though you remember singing it years ago. Cryptomnesia is why it can be hard to remember where you heard something; your brain might think it's a new memory instead of an old one.