CO-18: Duplicate claim or service
UPDATED JULY 2026 · U.S. PLANS · NOT LEGAL OR MEDICAL ADVICE
The payer believes this exact claim was already submitted. Often harmless — a resubmission crossing paths with the original.
CO-18 at a glance
- Code group: CO — Contractual Obligation — in-network providers generally cannot bill you for CO adjustments.
- Who usually fixes it: The provider’s billing office.
- Worth appealing? Rarely — usually fixed by resubmission.
What to do about a CO-18 denial
- Check whether the original claim was paid; if so, nothing to do.
- If the original was actually denied or lost, the billing office should reference it and resubmit properly.
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