A lot of times a producer will double the vocals. But the vocalist must be good at it. Emenim is a good example of someone who is very good at doubling. But its not really necessary to make the vocals sound 'full'. It gives the vocals an interesting sound, but what really makes vocals sound 'full' is using a high quality preamp, a nice vocal mic, and the right compression and reverb. Those are the most important things for getting a good vocal sound. Doubling, is more of an artistic decision. For other things like acoustic guitar, I will use a stereo pair to give the sound more dimention, but thats different than doubling. You can always record something with 2 mics for a stereo image, and if you dont like it later, just mute one of them. If you are looking for some serious advice on recording techniques, check out
http://www.digidesign.com/ and go to 'support' and then to the 'user conference'. If you use the search function, you can find a wealth of information from top engineers. Good luck.