What CO-18 means
The payer flagged this claim as a duplicate of one already adjudicated. Often a false positive — the same service was billed twice by mistake, or a corrected/resubmitted claim was not recognized as different from the original.
Common causes
- The same claim was submitted more than once
- A corrected claim was not submitted with the right frequency code
- A bilateral or repeated service appears identical without a distinguishing modifier
How to appeal CO-18
If the denial is correct, no action is needed. If the second submission was a corrected claim, resubmit using frequency code 7 (corrected) and reference the original claim number. If services were genuinely distinct (e.g., bilateral procedures), add the appropriate modifier and document the separate services.
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