I can ruin any that were on Rock Band Refer to the time I made the vocals louder on Geek Stink Breath and totally murdered the song.
Billie has always double tracked, it's a perfectly common production technique, and a lot of the time he joins in with the harmonies that Mike does. The main difference on the Trilogy was that Mike just didn't join in. God, I miss them doing stuff like Having A Blast, where it's unclear whether Billie or Mike is taking the lead. Oh, I guess that happened with the title track of Breakdown, but whatever, that's one song.