its normal. as colin has said, if you're right handed, your right bicep is probably going to be bigger, same for shoulders, calves etc...
look at arnie.
dont get this mixed up with uneven shapes of your muscles. often your biceps will be equal in size but one will appear bigger than the other because of a slightly different shape. the best way to see if this is the case (rather than one being bigger than the other) see if the one that is smaller is just as strong as the bigger one
if one is bigger than the other then the best way to correct it is by training as heavy as the weakest one allows. eventually they'll even out. so if for example you can do 12 reps of X kg on your right arm but only 10 on the other, then do 10 on both. failure on both arms at different reps is only going to make it worse.
dont do an extra set either. its difficult to know whether an extra set will make it bigger, or weaker (overtraining).