If you were earning income from a second job prior to being let go, and you continued to earn the same amount of income from the second job after you were let go, that amount would not be included in the calculation of your earnings for the purpose of calculating damages.
However, if you used your second job to supplement your earnings after you were terminated, for example by taking on additional hours, then the additional income you earn will mitigate, or decrease the damages you are entitled to.
For example, if you earned $200 per week at a second job prior to being let go and were able to increase your earnings to $600 per week as a result of being let go, the damages you are entitled to will be decreased by $400 per week (current earnings of $600 minus the previous earnings of $200).