No, the fact that you incurred additional expenses does not entitle you to keep the extra money unless those extra expenses are ones which under the plain terms of your policy and coverage, the insurer was obligated to pay...and even then, if the insurer disagrees with your interpretation, you can expect that they may take legal action for its return and you'll have to prove your entitlement to the money in court. But if the check truly was a mistake and you are not owed that money under the terms of the policy, you will have to return. A mistake does not legally entitle you to money.