If I purchased a home alarm and was double billed as a result of a mistake on the part of the vendor, is it liable to me for the over billed amounts?

The question I have is why did you double pay for so long on the monitoring portion of your home alarm system where two companies were doing the work of one? Were you sending two separate checks to each of the monitoring companies each month?

If so, why? My concern with your question is that the two companies were actually doing services for you during this eight year period, but you did not cancel one of the services until recently. You received the benefits of these two companies.

You received some years back notice that the vendor for your system had assigned the monitoring of the system to a new company but did not advise the old company to cease work when you signed the contract with the new monitoring company. Neither did the company that installed the system.

The answer to your question as to who owes the excess payments by you of roughly $3,000.00 lies in the original contract with the home alarm system with you. Read it carefully as to any mention as to obligations and duties concerning the monitoring of the system and notices. 

If there is no mention, it seems that both you and the home alarm company that initially installed the system are at fault for the double charges for what happended after such a long period of time.