I use Antares Vocal Doubler plugin in my Sonar. Singing the same track twice is sometimes very hard to do. It's hard to repeat the nuances. I suppose you could copy the lead vocal over to another track and EQ differently to get a doubling effect. Seems like a lot of work. In my case I rarely sing a song the same way twice. Especially if it's on a different day and I may feel different. Plus why sing out of tune twice LOL

Just kidding so please don't take offense. But in all honesty I found that the Antares or any doubler plugin (someone mentioned WAVES) will fill the bill and they are pretty inexpensive.