Synchrony Bank Debt Collection Complaint

Synchrony Bank Debt collection department,

Credit card False statements or representation Attempted to collect wrong amount California

I was sent a letter to owe the amount of $51.00 on my BANANA REPUBLIC credit card account. I paid off all amount that was due on that credit card and it currently shows on my online account that I have no amount due at this time. The last payment I paid in full was in - -, 2015. I do not understand why I am receiving a letter from the collection department for an amount that I did not even spend. Please fix this false statement.