I definitely would NOT recommend going lighter on one side or doing more reps with the stronger side, which would accentuate the strength imbalance. You goal should be to try to balance strength on both sides.
The best options:
1) stop when the weaker side has had enough
2) on moves like concentration curls, where you can do self-spotting and assistance, start with the weaker arm, do as many reps as you can, then do a couple of negative assisted reps (help lift the weight with the other hand, using as little help as necessary, then lower slowly with just the one arm)
3) start with the weaker arm, then do the stronger arm (same number of reps) then go back to the weaker arm and do an additional set