If you are going down the double tracking road, be very careful of the ends of words. Try experimenting with not pronouncing "P" or "S" sounds as these help define where words start and stop. This gives you a little more wiggle room in terms of recording an exact second copy.
Another thing to try is cutting and pasting the original vocal track into a second track. Having done this try adding plugin type effects to the second track and play it back along with the original and see if it helps.
Good luck !
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my toons Brought to you by Dr BadGAS
my toons Brought to you by Dr BadGAS