The quick and dirty way to do this is to clone the tracks in the methods already described.
The right way to d it is to record the track multiple times. Take the time to get the tracks (as many as you need.... at least 3 tracks) as close to perfect as you can get them.
Listen to the Beatles. They perfected the vocal doubling technique. Sir Paul and John would at times spend days upon end just working on the vocal track to a 2 minute 30 second song getting the parts pitch and timing perfect. Listen to their music.
Then once you have the tracks pick the lead and the others will be the doubling tracks. No need to time shift them or pitch shift them.... matter of fact you want them to be as close to the original lead track as is possible. Simply set the levels properly and pan them properly and you should have a nicely doubled track without having it sound processed and fake.
Try the same thing with the harmony tracks in the project too.
this project>>>
http://soundclick.com/share.cfm?id=11962059 had a bunch of vocal tracks. 2 leads, and IIRC 2 doubling tracks on each of those.... (that's 6 on lead vox) and then harmonies... IIRC there were a number of tracks for harmonies. ( at least 2 each for 4 more) Lots of true doubling in that on the vox. Nothing is cloned, it was all recorded track by track.
Take the time to do it right and you will appreciate the sound a whole lot better.