Double+ tracking helps with giving the guitars more punch and 'oomph' in the mix. Clarity is achieved by panning, EQing, less gain on the amp and good timing. It's best to play to a click when you're double tracking, as otherwise it's very hard to get it perfectly in time. If you don't reduce the gain, then it'll end up sounding like a bit of a mess and really nasty in that high-end. You want to take enough gain down to just chop that fizz in at the top... try playing with the presence and treble controls too.
Pantera is a classic example of multi-tracked guitars... lots of tracks there :)
