CO-18 Denial Code
What it means
The insurer thinks this exact claim was already submitted, so it rejected the second one as a duplicate.
Why it happens
The provider submitted the claim twice, or an original claim was still processing when a second was sent.
What to do
Usually a provider-side issue. Confirm with the billing office whether the original was paid; if it wasn't truly a duplicate, they resubmit with documentation. You typically owe nothing here.
Can you appeal it?
Yes, if it was wrongly flagged as duplicate.
Official description
Exact duplicate claim or service.
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