A similar trick I've used for "quick and dirty" doubling vocals after the vocalist has gone home is to duplicate the track and then run the second track through Melodyne (or something similar) to quantize the timing and correct any minor pitching mistakes... it can quite convincingly give the effect of a real second vocal take once balanced to the original track (also good if you're using it on your own vocals when you're as "adequate" a singer as I am
).