RJM scam

Hi I just received another letter from RJM Acquisitions llc about an old debt that I had payed off two yrs ago , the first letter I had forgot about because I thought it must be a mistake and I'l deal with it well like I said I forgot about it never received a call from any debt collectors like you normally would ,then today I received another letter from RJM they never mention the first letter this one like the last one offers a free gift if I pay in full ( never heard of any debt collection company doing this) so right away a redflag went up I typed the name in on google and popped up all these people who have been scammed by this company ! ok so with all that said what do I do where do I start please help how do I beat RJM???