Another thing, Mike might not actually know he's using a idea from another song, at least as soon as he writes it. I know that when I write music, I'll write a whole song, then when I listen to it, I'll be like "hey, you know, that small part sounds a lil like this song I think I've heard before" and pretty much, its like the other song was a subconcious memory that I didn't know I was remembering at the time I was writing the song.