What is CO 18 Mean?
The CO 18 denial code stands for “Duplicate claim/service.”
This means the insurance payer has determined that the claim or a specific service line was already submitted and processed previously — and the new claim appears to be a duplicate.
In other words, the payer believes you’ve already billed for the same service, same date, and same provider — so they deny the additional claim.
Common Reasons for CO 18 Denials
The CO 18 denial code occurs when a payer identifies that a claim is an exact duplicate of one already received or processed. Understanding the causes behind this denial helps prevent costly delays and rework.
Here are the most common reasons your claim might be flagged as a duplicate:
1. Duplicate Submission
Sometimes, the same claim is accidentally sent twice — either due to a billing system error or a resubmission before the first claim was processed.
💡 Tip: Always check claim status before resubmitting. Use your clearinghouse or payer portal to confirm if a claim is already on file.
2. Missing or Incorrect Modifiers
When a provider performs the same service multiple times on the same day, you must use the correct modifier to show that each service is valid and separate.
For example:
Use modifier 76 when the same provider repeats a procedure.
Use modifier 77 when a different provider repeats the same service.
💡 Tip: Missing these modifiers can make it appear like a duplicate claim, triggering an unnecessary denial.
3. Same Service, Same Day — Different Provider Not Mentioned
If two or more providers perform the same procedure on the same day, but the claim doesn’t clearly indicate this, the payer’s system assumes it’s a duplicate.
💡 Tip: Always include the rendering provider details or modifier 77 to show it’s a legitimate separate service.
On-Call Scenario: CO 18 – Claim Denied as Duplicate
Here’s the On-Call Scenario for CO 18 Denial:
Rep:
The claim was denied as a duplicate.
You:
Thank you. May I please have the denial date?
↓
Now, check if the CPT code was billed more than once for the same date of service (DOS).
If the CPT was not billed more than once:
Could you please check the status of the original claim?
→ Then proceed based on the original claim’s current status using Ar Scenarios.
If the CPT was billed more than once:
Check if the modifier, rendering provider, and exam time in the medical records are the same or different.
If they are different:
The claim seems to have been billed correctly with differences in modifier, provider, or exam time. Could you please reprocess the claim?
If the rep agrees:
Thank you. What is the turnaround time (TAT) for reprocessing?
May I have the claim number and call reference number, please?If the rep refuses:
If asked to send a corrected claim:
No problem. What is the time frame for submitting the corrected claim?
May I have the claim number and call reference number, please?If asked to submit an appeal:
Could you please share the fax number or mailing address for sending the appeal?
What is the time limit for appeal submission?
May I have the claim number and call reference number, please?
If they are the same:
Since all details are identical, it appears to be a true duplicate.
Follow the payer’s standard duplicate claim policy or your AR workflow for next steps.
Important Notes & Actions (CO 18 Denial)
Please take action according to your current process update.
The steps below may vary slightly depending on your team’s workflow.
1. When the same CPT is not billed more than once on the same DOS
Ask for the original claim status.
Follow the AR scenario based on the denial reason.
2. When the same CPT is billed more than once on the same DOS
a. If the modifier, rendering provider, or exam time are different:
If the rep agrees to reprocess, note the turnaround time (TAT) and set a follow-up for that date.
If the rep denies reprocessing and asks for a corrected claim, do the following:
Update the correct modifier.
Resubmit the corrected claim with billing code “7” and include the original claim number.
b. Modifier guidance for corrected claims
When different providers performed the same service → Add modifier 77 and resubmit the corrected claim.
When the same provider performed the same service at different times → Add modifier 76 and resubmit the corrected claim.
3. If the corrected claim is denied again for CO 18
If the insurance rep still refuses to reprocess → Submit an appeal with all supporting documents.
4. When modifiers on the charges are different
If the rep still denies reprocessing → Send an appeal to the payer.
5. For Medicare
Do not send a corrected claim.
Submit a new (fresh) claim instead.
6. When asked to submit an appeal
Calculate the appeal filing limit based on the denial date.
If the time limit is open, send the appeal with documents.
If the time limit has expired, write off the claim (unless the client requests otherwise).
If the client requests to send documents even after the time limit is crossed, follow client instructions.
7. When modifier, provider, and exam time are the same
The charge is a true duplicate and should be voided.
8. When clarification is needed
If there is any doubt about whether the charges are duplicates, get confirmation from the coding team before taking further action.
Click here for more AR Scenarios and Denials.