Led Zeppelin had, let's say, a somewhat checkered past of "borrowing" riffs, chord progressions and lyrics. Whether it was intentional or not, who knows?
But to be fair, who here can honestly say they haven't jammed out an awesome 'new' riff or even entire song before realising, days later, that it was actually an existing track you heard on the radio the week before last? I do it more than I'd like to admit.