I sometimes double track, obvs for stuff like harmonies (playing stuff an octave higher turned down really low in the mix always sounds awesome)
just double tracking a whole solo (especially if it's shreddy) hides a lot of mistakes and generally sloppy players do this. Like 'Sinister Mephisto' said, most the shadows fall solos i can remember are double tracked AND zakkk wylde does a lot of double tracking, but i'm not saying those dudes are sloppy. Can sound really cool esp if panned hard left and right