When is enough?

Last year of April I paid off my debt to metcredit, granted after a long winded argument of thinking I only had one pillar of debt, but nope after that I finally paid the second pillar of debt and since a year has nearly passed without emails or phone calls, I figured all was good.... I get a letter from action collection & receivables management suddenly saying that same debt is still owed... but after all that with metcredit? I still owe the same debt... that i just finished paying in april? Where the debt should've been loong dead and decomposed and forgotten about? What are the appropriate steps to give them clarity this shouldn't have been even at the table? Cause im befuddled at this point