When an insurer pays defense costs for a claim that also triggers the duty to defend under another insurer's policy, it has a claim for equitable contribution against the other insurer. This happens, for example, when an insured has purchased overlapping coverage, or when there are multiple insureds being sued in the same action (say, a property owner and a contractor who are additional insureds under each other's coverage), or when there is a continuous injury (say, an expanding plume of pollution) that is deemed to trigger policies issued over a span of years.