CO-18 Denial Code: Exact Duplicate Claim or Service

Payers match incoming claims against claims they have already received. When every key element matches, the new claim is denied with CARC 18. This most often happens when a claim is rebilled while the original is still processing, or when a correction is sent as a new original claim instead of a replacement.

Also appears as OA-18. The group code changes who is responsible, not the reason.

CO-18 means the payer already has a claim for the same patient, provider, date, and service, so it denied this one as a duplicate. Before doing anything, check the status of the original claim. The fix is usually to work the original, not to resubmit again.

Common causes of CO-18.

• Rebilling a claim that was still in process instead of checking status
• Sending a corrected claim with frequency code 1 (original) instead of 7 (replacement)
• Repeat procedures on the same day billed without a repeat modifier such as 76, 77, or 91
• Two providers in the same group billing the same service
• Clearinghouse or batch resubmission errors

How to work a CO-18 denial.

The order matters: confirm the facts before deciding between a corrected claim, a reconsideration, or an adjustment.

1. 1Check claim status on the original submission through the payer portal or a 276/277 inquiry.
2. 2If the original was paid, close the duplicate with no further action.
3. 3If the original was denied, work that denial reason instead.
4. 4If the services were genuinely separate, submit a corrected claim with the appropriate modifier and documentation.
5. 5For corrections, always use frequency code 7 and the original claim number.

How to keep CO-18 from coming back.

• Check status before rebilling any claim with no response
• Set a follow-up cadence based on payer processing times instead of automatic rebills
• Train billers on corrected-claim frequency codes

Fixing this at the source usually sits with ar follow-up specialists →

Questions about CO-18.

Denials that often travel with this one.

Browse all denial codes →

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