If you listen closely in alot of rap, the verses aren't really doubled. It's usually doubled on the chours, and for a verse its a single take and an ad-lib take. However, on some other rap style, they double all the way through. This can have a very good result if you get the takes perfectly recorded. If the artist slips off track on one or the other take, it doesnt sound as good. Depending on what artist Im working with determines what technique I use. However, it seems pretty logical to have a single take and adlib for a verse, and double the chorus, to sort of define each of them so they don't just blend together.