When an injured plaintiff has received collateral compensation from insurance, a gift, or other sources (such as the expense borne by the preferred providers, which wrote off a portion of their bills pursuant to the PPO contract), allowing a deduction for damages in that amount would result in a windfall for the tortfeasor and underpayment for the injury. (Helfend, supra, 2 Cal.3d at p. 10; Arambu