Repetition has a strange relationship with the mind. Take the experience of déjà vu, when we wrongly believe we have experienced a novel situation in the past – leaving us with an spooky sense of pastness.But we have discovered that déjà vu is actually a window into the workings of our memory system.Our research found that the phenomenon arises when the part of the brain which detects familiarity de-synchronises with reality. Déjà vu is the signal which alerts you to this weirdness: it is a type of "fact checking" for the memory system.But repetition can do something even more uncanny...