Bad Service

These people have been emailing me, made time paid , emailed them proof of payment, tried calling no response, emailed them and still no response, they have now emailed me about being summoned to court because of an outstanding payment , and mind you , I received an email from them thanking me for payment during the week and I have forwarded the same email asking what must I do because I have paid. Am I being harassed because this is a legal company? Is this how these debt collecting companies work? Funny how they keep emailing me but not even once are they responding to my mails