You might want to try using a large diaphragm condenser mic as opposed to the good old SM58. You can even get these as USB mics for about 100 bucks. They will help a lot. The '58 is a great live mic however it is not as sensitive as an LDC.

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 !