Listen to Lenny Kravitz's stuff. I know one of his riffs is a direct copy of LZ's Heartbreaker, or sounds like it anyway. This is really nothing new, though. You're bound to "borrow" from time to time, for "there is nothing new under the sun." How many times have you come up with what sounds like a cool riff or chord progression, only to have someone say, "Oh that sounds just like that Led Zeppelin song" or whatever? This really comes down to the philosophical paradox of invention: How can you come up with something "new," when, in order for it to make sense to people, it has to follow a familiar form? Things that make you say "Hmmmmmmmmm . . . . . ."