# CARC 18 Denial Code (Exact Duplicate Claim/Service) | ANSI 835 EDI Spec
> Comprehensive healthcare EDI technical reference and denial resolution guide for Claim Adjustment Reason Code 18 (Exact Duplicate Claim/Service) under ANSI ASC X12 835 005010X221A1.

- **Canonical URL:** https://stanzaapi.com/tools/x12-parser/carc/carc-18-duplicate-claim
- **Governing Standard:** ANSI ASC X12 Standards (005010) (Accredited Standards Committee X12 / HIPAA 5010)
- **Edge Latency SLA:** <2ms Edge (Cloudflare V8 Isolates)
- **Execution Model:** Pure compute $f(x) = y$, 100% In-Memory (Zero Store, Zero Log)

## Technical Overview & Governing Rules
Claim Adjustment Reason Code (CARC) 18 communicates: "Exact duplicate claim/service.".

In ANSI X12 835 Electronic Remittance Advice (ERA) files, CARC 18 is reported in Loop 2110 (Service Payment Information) or Loop 2100 (Claim Payment Information) under the Claim Adjustment (CAS) segment with Group Code 'OA' (Other Adjustment), formatted as: CAS*OA*18*AMOUNT~.

Root Cause: A claim with the same patient, provider, date of service, procedure code, and billed amount was previously received and adjudicated.

Adjudication & Appeal Strategy: Verify whether the earlier claim was already paid. If services were legitimately repeated on the same date, append modifier 76 (repeat procedure) or 59/XE and appeal.

## Specification Matrix
| Attribute | Value | Technical Guidance |
| --- | --- | --- |
| Reason Code | CARC 18 | Exact Duplicate Claim/Service |
| Standard Group Code | OA | Other Adjustment |
| ANSI 835 Segment | CAS*OA*18*AMOUNT~ | Loop 2110 or Loop 2100 |
| Balance Billing Patient | Prohibited. | Compliance with billing regulations |
| Common RARCs | N522, MA01 | Remittance Advice Remark Codes |
| Recommended Action | Verify whether the earlier claim was already paid. If services were legitimately repeated on the same date, append modifier 76 (repeat procedure) or 59/XE and appeal. | Workflow for billing teams |

## Standard JSON Response Model
```json
{
  "transactionType": "835",
  "claimNumber": "CLM-2026-001",
  "adjudication": {
    "billedAmount": 500,
    "paidAmount": 0,
    "adjustments": [
      {
        "groupCode": "OA",
        "groupName": "Other Adjustment",
        "reasonCode": "18",
        "reasonDescription": "Exact Duplicate Claim/Service",
        "adjustmentAmount": 500
      }
    ]
  }
}
```

## Frequently Asked Questions & Edge Cases
### Q: What does CARC 18 mean on an 835 ERA?
**A:** CARC 18 signifies: "Exact duplicate claim/service.". It explains why the payer did not pay the full billed charge.

### Q: Can a provider balance-bill the patient for CARC 18?
**A:** Prohibited.

### Q: How do medical billing teams resolve and appeal CARC 18?
**A:** Verify whether the earlier claim was already paid. If services were legitimately repeated on the same date, append modifier 76 (repeat procedure) or 59/XE and appeal.