#1
Have you ever heard a band steal another band's riff?

Ozzy Osbourne - Countdown's Begun intro is very reminiscent of Hells Bells.

Metallica - For Whom the Bell Tolls has Black Sabbath's Fairies Wear Boots outro riff in a different position.

Metal Church - Little Boy uses the intro riff of Black Sabbath's Sleeping Village, which is a very awesome riff if you ask anyone.

These are the top 3 that I caught.
#3
dude go to you tube and type metal that sounds like other metal....trust me is worth it there 20 something episodes men look it up
#4
most guitars have what, 22 frets?
im not doing the math, lets let someone else do that
how many songs have been written and recorded in the past twenty years
fourty?
sixty?
people are bound to repeat totally on accident, and some people take riffs and make them their own, no shame in that
I've got a friend who wrote a riff, then when sacrament by lamb of god came out
that riff was on it
weird
#5
ok I swear to god paranoid stolen from that riff in Dazed and Confused that's about 2 minutes in.