That kinda stuff like the Maiden's Genghis Khan riff is bound to be found everywhere since it's a short and relatively simple progression. Might be rip-off but then again it might not be. And in the end copying some parts of pieces of music is (intentionally or not) is inevitable and in no way harms the original. Classical music is based basically on a few popular composers who ripped all their colleagues off but did it well. Of course they have written great stuff themselves but for example Brahms's probably most famous piece of music, the Hungarian Dance number 5 was based on the csárdás by Kéler Béla titled "Bártfai emlék" which Brahms mistakenly thought was a traditional folksong. But then again Brahms orchestrated it himself. Of course there have been some truly great minds who have been so much ahead of their peers that they didn't copy. Bach, Mozart and Beethoven are some of those guys.