CO-18: Exact duplicate claim/service (Use only with Group Code OA except where state workers' compensation regulations requires CO)
The payer's system identified this claim or line as an exact match to one already submitted and processed. Nothing new was paid because it looks like the same claim was sent twice.
What the group code means
- CO
- Contractual Obligation. The provider is responsible and the amount cannot be billed to the patient.
This CARC is reported under more than one group code depending on the payer and the circumstances: OA, CO.
Why it fires
- The claim was resubmitted before the original finished adjudicating, creating a timing duplicate
- A clearinghouse or billing system glitch sent the same claim multiple times
- A corrected claim was submitted without the required frequency code, so the payer saw it as a duplicate rather than a correction
Corrective actions, ranked
- 1
Confirm it is truly a duplicate
Check the original claim's status. If it is still in process, wait for it to finalize rather than resubmitting.
- 2
Use proper correction coding
If you meant to correct the original claim, resubmit with the correct frequency code (7 for replacement, 8 for void) and reference the original claim number.
- 3
Fix the submission workflow
If your clearinghouse or EHR is auto-resubmitting claims, correct the trigger causing the duplicate transmission.
Is it worth appealing
Not appealable if it is genuinely a duplicate. If the payer mistakenly flagged a legitimate corrected or separate claim as a duplicate, submit a corrected claim with the proper frequency code instead.
Retrieved 2026-07-18.
Do not let this happen again
Scrub the claim against NCCI PTP and MUE edits before you submit, free and in your browser.
DenialPath provides billing and administrative guidance based on published CMS and X12 sources. It is not medical advice, not a coverage determination, and not a guarantee of payment. NCCI and MUE edits are republished quarterly and payer policies vary by contract. Always confirm against the payer's own current policy before submitting or appealing.