Denial code CO-18: Duplicate claim
The payer already received this claim or service.
At a glance
| Code | CO-18 (group CO, reason 18) |
| Who owes it | Contractual obligation: The provider must write this off. The patient cannot be billed for it. |
| Type | Duplicate claim |
| Worth appealing? | Usually not |
How to fix it
- Check claim status: the first claim may still be processing or already paid.
- If this is a different service, add modifier 76/77 or 59 and send a corrected claim.
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Start free for 14 daysFrequently asked questions
What does denial code CO-18 mean?
The payer already received this claim or service.
How do I fix a CO-18 denial?
1. Check claim status: the first claim may still be processing or already paid. 2. If this is a different service, add modifier 76/77 or 59 and send a corrected claim.
Should I appeal a CO-18 denial?
Usually not. CO-18 is normally an expected adjustment or a duplicate. Confirm the amount is right, then post it.
Can I bill the patient for CO-18?
No. With group code CO the amount is not the patient's responsibility and must be written off unless it is overturned.
Plain-English summary for billing staff, not the official code text. See x12.org/codes for the official wording, and always follow the payer's own policy.