Oh God, Not Again! By Sarah1281

Adult Harry Potter accidentally goes back in time and wakes up on his 11th birthday again, but with all his memories of the future intact.

Rebellion

Harry starts up a Rebellion after Dumbledore is forced to flee Hogwarts and his two targets are Dolores Umbridge and Severus Snape. He has a foolproof plan: to drive them insane by acting insane himself.