CARC CO-18: Duplicate Claim/Service — DME Fix Guide

CO-18 means the payer has already processed a claim for this patient, this service, this provider, and this date of service. They're denying the second as a duplicate. The denial can be legitimate (you did send it twice) or a payer error (they matched on insufficient criteria).

• Accidental resubmission. Billing team submitted the claim, didn't see it in the system, and submitted again. Both hit. This is the most common cause.
• Monthly rental re-billing. For rental equipment (oxygen, CPAP), the billing cadence must match the delivery cycle exactly. If dates overlap by even one day, the second month bills as a "duplicate" of the first.
• Replacement equipment not flagged as replacement. A replacement CPAP unit uses the same HCPCS as the original. Without a KX or RA/RB modifier indicating replacement, the payer matches it to the prior claim.
• Clearinghouse resubmission after timeout. Clearinghouse submitted the claim, the response timed out, the system re-queued and resubmitted. Two claims reach the payer.

1. Pull both the denied claim and the original claim. Compare every field: patient, dates, HCPCS, provider NPI, charge. If they're identical, the denial is legitimate — do not resubmit, check the original claim's status.
2. If the original claim was paid: Accept the CO-18 denial. The first claim was correctly adjudicated.
3. If the original claim was rejected or not found: This is a payer error. Document the original claim's rejection or non-receipt and appeal.
4. If it's a replacement item: Resubmit with proper replacement modifiers (RA = replacement of a lost/stolen item, RB = replacement of a worn/damaged item) and a physician order for the replacement.
5. If it's rental re-billing: Confirm the billing cycle dates don't overlap. Adjust the service date span and resubmit.