I received a CP503 bill stating it was my 2nd notice, but I never received a 1st notice. I received a refund for 2023 of $5000. Then 6 months later, I get a bill for over $5000. All of the math adds up. There appears to be no reason I owed this money. I've contacted the IRS several times and have gotten nowhere. I've already paid the bill in fear of garnishments or liens. Who do I contact or how do I go about addressing this?