First and foremost, I'm going to have the artist spit a main, a stress, and an adlib. If it comes out sounding ok, but sounds like it needs more...I will have them add a second main to see if it thickens the vocals up any. Since I'm not going to be mixing the song RIGHT THEN AND THERE, I will save it (using option C), and work with it.
Now, I would make my decision based on how the song sounds when I mix it down. If I have 2 vocal tracks, each slightly panned to the right and left, and it sounds too busy...I'm going to try centering each track and lowering the second...and see how that sounds. If it sounds shitty, I'll mute the second...and if that sounds better, I'll delete it altogether.
There are too many things to take into consideration before I can REALLY answer this question. Does the rappers voice sound like MINE? Or does it sound like Too Short? 'Cause if the dude has that deep ass Sydal type voice, doubling it will probably sound like shit...because my voice is thick as fuck as it is. BUT, if the artist has a high pitch and his voice naturally comes out thin, something has to be done to it in order for the vocal to be up there with the rest of the track.
It also depends on what type of mic is being used. Some mics have that warm sound already and a 2nd main might not be needed. A lot of things to take into consideration in any scenario like this. Obviously, vocals recorded on an SM57 aren't going to be as thick and crisp as they would on a Neumann.