Let's see ... Pro Tools... Headphone out the interface with the output/input knob set pretty balanced so you actually hear your input being on time? You're hearing processing latency. You're only hearing 50% of the signal at zero latency. The rest is delayed slightly. That's why it sounds like multiple voices.
Try doing this: you record enable the track, right? Record enable and mute the output from that track. Now you will only hear yourself dry once through the headphones. If you want to hear yourself with a reverb (this helps some people -- I like this) you'll need to split the signal in an effects unit, make one dry, and the other with a reverb, and send both signals to the interface. Record only the dry. I prefer plugins for effects because they're non-destructive.
There are plugins available for vocals that have doublers, which would be for unison doubling but it adds some sort of delay and modulation. You could grab one of those. And if you want you could even get something like Melodyne and put that on a copy of the track, then drop the pitch of that track one octave for octave doubling. And with DNA in Melodyne you could even create other harmony voices.
Also you might want to use a delay on your vocal track and add a little reverb for warmth depending upon the genre you're doing.
