Received (and paid) bill for 2 months charge

I Hello.

I received a bill stating that I missed a payment for the previous month. Being in a hurry, and realizing that I might have missed it, I paid the double charge quickly. Now that I have a few extra minutes, I reviewed my billing statements and bank statements and realize that I DID in fact pay the previous month's bill. Why did this happen? How can I get reimbursed for the over payment? How does someone contact the billing department directly?

Thank you.