what is it you want to do exactly?
i think i've heard of a pedal that you can play a riff or wahtever....and it'll keep repeating it while you play something else over it...
but usually...."overdubbing"...is playing something new over an existing track so that you can try out other "takes"...to see how it sounds...if you like it...you use the overdub...or punch it in...in your final recording..

or i guess some people might use 'overdubbing' and 'doubling' as the same thing..
i think of 'doubling' as playing a guitar line, for example..and actually recording another track of guitar that matched it exactly...thus...doubling the line...smashing pumpkins did this alot on the siamese dream cd..whatever that was called.....

sooo maybe you could let us know what you're after...
good luck
-G