"Oh God, Not Again" by Sarah1281 on Fanfiction.Net. Yes, I know. This was finished in 2009 so it's practically vintage by internet standards. But the gist of it is: Harry Potter wakes up in his 11-year-old body, back at the start of the series, but with all his memories of everything that happened. All his skills. And the relative maturity of his age, which means he decides this time he's going to do a better job of things. Including mouthing off to people he wasn't afraid of, making friends of Neville and others earlier, and so on.
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