There are a couple of free Automatic Double Track (ADT) plugins around that'll do that for you.
Try looking for airwindows for a start.
Singing a double is always the best answer but also the hardest to do right.
To get that effect you're after the simplest way is to duplicate, add some wow & flutter plus the tiniest amount of delay through a plug in and blend the effected track into the original slowly until it is audible and then pull it back a little.
OR
Duplicate & heavily compress, high pass above 200 add a bunch of high end EQ and then blend as above. That is based on the Motown Exciter Compressor - it adds "life" to the vocal without getting fat or loud.
Exciter

Cheers
rayc
"What's so funny about peace, love & understanding?" - N.Lowe