Denial codes / CO-18
CO-18 denial: meaning, fix & appeal letter template
Payer thinks this service was already billed.
What it means
Official CARC 18 descriptionExact duplicate claim/service.
The payer matched this line to another claim with the same patient, date of service, provider, and code. Either it really is a duplicate, or two distinct services weren't distinguished on the claim.
Common causes
- Claim resubmitted while the original was still processing
- New claim sent instead of a corrected claim (frequency code 7)
- Repeat procedure on the same day billed without -76/-77
- Distinct procedures billed without -59/-XS/-XU/-XE/-XP
- Bilateral or multiple units split across lines incorrectly
How to fix it
- Check claim history: was the original paid, denied, or pending?
- If it was a true duplicate, no action — close the line
- If correcting a prior claim, resubmit with frequency code 7, not as a new claim
- If services were genuinely distinct, add the right modifier (76, 77, 59/X{EPSU}) and resubmit
- Hold resubmissions until the payer's processing window has passed
Appeal or corrected claim?
Usually no appeal. If it was a true duplicate, close it. If you meant to correct a claim, send a corrected claim (frequency 7). Appeal only when the services were genuinely distinct — e.g. a repeat procedure (-76/-77) or separate session/site (-59/-XS/-XU) — and documentation shows two separate services.
What to attach
- Documentation of both services (times, sites, or separate encounters)
- Corrected claim with the distinguishing modifier
- Claim history showing the original claim number and status
CO-18 appeal letter template
Replace every [BRACKETED] field. Remove any optional paragraph that doesn't apply.
[PRACTICE NAME] [ADDRESS] · [PHONE] · NPI [NPI] · TIN [TIN] [DATE] [PAYER NAME] Attn: Appeals / Provider Disputes [PAYER APPEALS ADDRESS] Re: Request for Reconsideration — Claim [CLAIM NUMBER] Patient: [PATIENT NAME] Member ID: [MEMBER ID] DOB: [DOB] Date of Service: [DOS] CPT: [CPT CODE] Billed: [AMOUNT] Denial: CO-18 — Exact duplicate claim/service. Dear Appeals Reviewer, We request reconsideration of the denial of the above claim. The above service was denied with CARC 18 as an exact duplicate. This service was not a duplicate: CPT [CPT CODE] was performed [TWICE on [DATE] / at a separate site / at a separate session] as documented in the enclosed [NOTES] (times [TIME 1] and [TIME 2]). The second service has been billed with modifier [76/77/59/XU/XS] to identify it as distinct, consistent with CPT guidelines and [PAYER NAME] policy. Based on the enclosed documentation, we request that [PAYER NAME] overturn this denial and reprocess the claim for payment in accordance with the member's benefits and our participation agreement. Enclosures: - Documentation of both services (times, sites, or separate encounters) - Corrected claim with the distinguishing modifier - Claim history showing the original claim number and status Please contact [CONTACT NAME] at [PHONE] or [EMAIL] with any questions. Sincerely, [NAME], [TITLE] [PRACTICE NAME]
Stop writing these off — get a free denial audit
Send us your CO-18 denials (and the rest). DenialIQ's agents read each one, pull the payer policy, and draft an appeal like this for your team to approve.