Report this immediately. You cannot be held liable unless you let it occur. Since you have notice, then it is arguable if it happens again, you are alowing it to happen.
As Ms. Henley stated above, you now have notice that someone is trying to ride your coat tails, so to speak. You need to notify the port that you did not authorize or sign the purported letter, in writing, and in a way that you can document it was sent and received (such as certified mail). You should put the other company on notice of your non-consent to sharing your insurance coverage. You may also need to notify your carrier so if someone tries to make a claim on your policy, they are on alert.
This response does not constitute legal advice. The facts and circumstances of any particular situation are unique and require an individual consultation before relying upon the general information contained here. The responding attorney welcomes the opportunity to consult with new clients to determine the appropriate direction and strategy for their case.