21 Guns (among other songs) grinds on my respect for Green Day.
21 Guns = Electric Light Orchestra - Telephone Line
American Idiot = Dillinger Four - Doublewhiskeycokenoice
Warning = The Kinks - Picture Book
Also, Boulevard of Broken Dreams bears a striking resemblance to Wonderwall by Oasis (though that's a bit different since that chord progression is overused in general).
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