EOB denial code CO-18: what it means

What it means

The insurer believes this exact claim or service was already submitted, so it was not paid a second time.

Contractual Obligation — an adjustment the provider agreed to by contract. The patient is generally not billed for CO amounts.

Official X12 description

“Exact duplicate claim/service”

Typical cause

The same claim was sent twice, or a corrected claim was read as a duplicate of the original.

What to do next

1. 1Confirm the original claim was actually paid — sometimes a legitimate second service is mistaken for a duplicate.
2. 2If two genuinely different services were performed, ask your provider to resubmit with documentation distinguishing them.