Please clear my name

I settled my Foschini account on 26 March 2016 in Hemingways when I asked for paid up letter they gave me a number to call to get the letter but they didn't tell me they handed the account over to attorneys.The attorneys started calling me saying I owe Foschini I told them I settled in-store but they never resolved the issue.Up to this day they still call for this account.My credit record is bad because of this account.What should I do? I can't settle 1 account twice and Foschini was not suppose to accept my payment if they already handed me over.