I am impressed you were able to get the refund back! What the insurer will likely state is that every year, every renewal you saw the coverage amounts, details of limits and accepted them by paying your premium. For this reason, they may be less than eager to go back and refund you for previous years since you de facto accepted it over and over and over again. Having said that, since they did refund you, they have accepted the mistake did appear and I would definitely reach out for a refund going back to the inception. It may be wise to enlist the assistance of an attorney to draft a convincing message.