CO-18 Denial Code: Duplicate Claim

The official definition

“Exact duplicate claim/service”

That is the verbatim definition of CARC 18 from the X12 Claim Adjustment Reason Code set, the standardized codes every insurer uses on EOBs and remittance advices. The letters in front of the number are the group code. CO: Contractual Obligation: an adjustment between the provider and the insurer. An in-network provider should not bill you for CO amounts.

What it means in plain English

The insurer says they already received and processed this exact claim, and this is a second copy. Usually that's true: a billing system submitted twice, or a corrected claim wasn't marked as a correction. The question that matters: was the original claim actually paid?

What to check on your EOB

• Your earlier EOBs: is there one showing this same claim already processed and paid?
• Whether the two claims are actually distinct services (different dates, different procedures) that only look like duplicates.
• Whether you're being billed for the denied duplicate. You shouldn't be, if the original was paid.

What to do next

1. If the original claim was paid: no action needed. The duplicate denial is correct, and you owe nothing extra.
2. If you can't find a paid original, call the insurer and ask them to trace both submissions.
3. If the services were genuinely separate, the provider needs to resubmit with documentation distinguishing them.

Who's responsible

Provider. Most denials carry a clear owner. Knowing whether the fix belongs to you, your doctor, or the billing office is half the battle. If it's the provider's error, you should not be paying for it.

Want the fundamentals first? Start with how to read an EOB and the 7 most common billing errors. This page is general information about standardized denial codes, not legal or medical advice.