Debt collection agency - Harris & Harris , Ltd of XXXX XXXX XXXX XXXX, XXXX XXXX, XXXX IL XXXX, telephone XXXX, sent me collection letter dated XXXX/XXXX/XX/XX/2015 demanding payment for debt that is already paid with original debtor. I sent a letter to collection agency dated XXXX/XXXX/XX/XX/2015 denying the demand was valid and stop any further collection attempt regarding the debt. The denial letter was mailed within 30 days of the demand letter. After the 30 days expired from the date of the demand letter, the collection agency sent a XXXX demand letter dated XXXX/XXXXXX/XX/2015 for the same debt. Collection agency did not acknowledge receiving my letter stating the debt was invalid and is continuing collection attempts for an invalid debt.