How to resolve a duplicate claim denial.

CARC 18 is rarely an appeal. It fires because more than one claim was submitted for the same service, for the same patient, on the same date of service. In most cases the right fix is a corrected claim with the right modifier, not an appeal letter. This guide walks through the resolution path first, and covers the narrow appeal case at the end.

Payer duplicate logic cannot tell a clerical resubmission apart from a legitimate second test, repeat procedure, or distinct same-day encounter. Billing teams that reflexively write off every CARC 18 line lose real revenue on services that were both medically necessary and properly documented. But teams that go straight to an appeal letter add weeks to a fix that a corrected claim would have paid in the next cycle.

One thing to check on the remittance itself: CARC 18 should arrive with group code OA (Other Adjustment), except where state workers' compensation regulations require CO (Contractual Obligation). If a payer sends CARC 18 with CO outside of that work comp exception, that miscoding is worth flagging with the payer — group codes drive how the adjustment posts and whether it looks like a write-off you agreed to.