It's pretty normal for one arm (usually the left if you're right handed) to be weaker than the other. That's why it's good to work with dumbbells, which can helo the weaker side catch up rather than allowing the stronger side to help it along.
I suggest starting the concentration curls on your stronger arm (which will give the weaker arm a bit longer to rest, but also set the goal for the weaker arm). Instead of usiing different weights, or doing a different number of reps (as poster 2 wondered about), I'd use this exercise as a great way to incorporate assisted lifting. Let's say you can do 12 reps at 12# for the right arm. Use 12# for the left arm as well, and once you reach failure (maybe you can only do 10), use your right arm to help get the weight to the up position, and lower the weight with the left arm by itself (working on the negative). Try to assist as little as possible, supporting your left hand right behind the wrist. Don't move up on weight until the left arm "catches up" to the right.
Have any of you found during the concentration bicep curls one arm is stronger than the other? I started the curls with my left arm using 12 lbs and found myself at failure half way through the set!! My right arm seems to be stronger even though I went down to 10 lbs for that.
