This is contribution. An example of this is if you have two insurance policies and are going on a business trip that is cancelled last minute. You can claim on the insurance the value of the trip- but only once. So you can either use one policy and not the other, or you can claim half the value of the trip on both policies. This stops people taking out multiple policies and then claiming on them all to make a profit. Legal Principles of Insurance is a known exam topic - this is from p.100-101 of the study guide