CO-18: Exact Duplicate Claim/Service

What Does CO-18 Mean?

With CO (Contractual Obligation), the CARC 18 adjustment for exact duplicate claim/service is a contractual reduction. The provider absorbs this amount per the payer contract or regulatory payment methodology. The patient is not responsible for the adjusted amount. Review the remittance to confirm the adjustment is consistent with your contract terms.

CARC 18 indicates that exact duplicate claim/service. The payer's system flagged this claim or service as a duplicate of a previously processed submission. Only one payment will be made for the same service on the same date.

Common scenarios that trigger this adjustment include: same claim submitted multiple times due to manual error or system timeout prompting resubmission; EDI system transmitted the claim twice due to timeout, connection error, or batch processing issue; Primary payer forwarded the claim as a crossover to secondary while the provider also submitted directly to the secondary. The group code paired with CARC 18 determines who bears the financial responsibility — CO places it on the provider as a contractual obligation, OA indicates a coordination of benefits or other payer adjustment, PR shifts it to the patient.

Common Causes

How to Resolve

1. Review the adjustment against contract terms Compare the CO-18 adjustment with your payer contract to confirm the reduction is consistent with agreed terms or regulatory methodology.
2. Verify the adjustment amount Confirm the dollar amount of the adjustment is calculated correctly based on the contracted rate and the service provided.
3. Appeal if the adjustment is incorrect Appeal if the claim was incorrectly flagged as a duplicate. Include the original submission timestamp (EDI acceptance report), the 997/999 acknowledgment, and explanation of why this is a separate legitimate claim. For Medicare, submit a Redetermination within 120 days of the RA date. Do not submit a new claim - use the appeal track.
4. Process the contractual adjustment If the adjustment is correct per contract terms, process it accordingly in your billing system. This amount cannot be transferred to the patient.

Common RARC Pairings

The RARC code tells you exactly what triggered the CO-18:

How to Prevent CO-18

• Implement a claim tracking system that flags potential duplicates before submission
• Wait for clearinghouse acknowledgment before resubmitting
• Use claim frequency code 7 for replacement claims, not new submissions
• Train staff to check claim status before resubmitting
• Integrate pre-submission duplicate checking in RCM software

Also Filed As

The same CARC 18 may appear with different Group Codes:

Related Denial Codes