CO-18: Duplicate Claim or Service
CO (Contractual Obligation)What is CO-18?
CO-18 is a Contractual Obligation code indicating the payer considers the claim a duplicate of a previously submitted and adjudicated claim. This code triggers when the same procedure code, date of service, and provider NPI appear on more than one claim submission.
Why Does CO-18 Occur?
- Claim resubmitted without a corrected claim indicator. The original claim was already paid or processed, and the resubmission was not flagged as a replacement (frequency code 7).
- Clearinghouse retransmission. The clearinghouse automatically retransmitted the claim after a timeout, creating a duplicate.
- Same service legitimately performed twice. The patient received the same procedure twice on the same day (e.g., bilateral injections) but the claim does not distinguish the two services.
- Payer system error. The payer's adjudication system flagged a claim as duplicate incorrectly.
How to Fix CO-18 Denials
- Check the original claim status. If it was already paid, no resubmission is needed. Write off or adjust the duplicate.
- If the services were legitimately performed twice, add modifier 76 (repeat procedure by same physician) or modifier 77 (repeat procedure by different physician) and resubmit.
- If the duplicate was a clearinghouse error, contact the clearinghouse to confirm the retransmission and file a corrected claim with frequency code 7.
- If the payer flagged it incorrectly, appeal with documentation showing the services are distinct (different times, different sites, or different providers).
CO-18 by Payer
| Payer | Common RARC | Appeal Deadline | Notes |
|---|---|---|---|
| UnitedHealthcare | N/A | 60 days from remittance | Check claim status on the UHC portal before resubmitting. |
| Anthem | N/A | 365 days from denial notice | Anthem's duplicate detection window is 30 days. |
| Aetna | N/A | 180 days from denial | Use frequency code 7 for corrected claims to avoid CO-18. |
| Cigna | N/A | 180 days from denial | Cigna flags duplicates based on CPT + DOS + NPI combination. |
| Medicare | N/A | 120 days (redetermination at MAC) | Medicare duplicate window is based on exact CPT, DOS, and NPI match. |
Related CARC Codes
If you are seeing CO-18, check these related codes: CO-16 (claim differs from payer records), CO-19 (claim status pending), CO-42 (claim timing issue).
Related CPT Modifiers
Modifier 76 (Repeat Procedure by Same Physician).
Common Questions About CO-18
How do I fix a CO-18 duplicate denial?
First confirm whether the original claim was already paid. If yes, no action needed. If the services were legitimately separate, add modifier 76 or 77 and resubmit. If the duplicate was a system error, file a corrected claim with frequency code 7.
What is frequency code 7?
Frequency code 7 (also called bill type replacement) tells the payer this is a corrected claim replacing a previous submission. It prevents the payer from flagging it as a duplicate.
Altair catches CO-18 denials before submission with duplicate claim detection. See how pre-submit claim scoring works.