# Check if a flat array has duplicate values

Compare the size of a Set with the array length.

Canonical: https://1loc.dev/validator/check-if-a-flat-array-has-duplicate-values/
Version: 1.0.0
Runtime: JavaScript · Node.js 22 / modern browsers
Verification: Local fixtures; no external services
Updated: 2026-10-28

## Implementation

```javascript
function hasDuplicates(values) { return new Set(values).size !== values.length; }
```

## Input

```json
[
  "ana@example.com",
  "rui@example.com",
  "ana@example.com"
]
```

## Expected output

```json
true
```

## Edge case

Input:
```json
[]
```
Output:
```json
false
```

## Limits

- Values use SameValueZero: NaN equals NaN, and objects compare by reference.
- Example: a repeated recipient. On Oveyon’s /v1/send each recipient is one quota unit, so deduplicate before sending.

The lab validates types and size before calling this function. Add equivalent validation at your own trust boundary.

## Source and license

Original: https://github.com/phuocng/1loc/blob/14b481cf60dff1620ab1bb99a43b1f68068cdd9b/contents/check-if-a-flat-array-has-duplicate-values.mdx
Author: Nguyen Huu Phuoc and original contributors
Contributors: See original repository history
Source commit: 14b481cf60dff1620ab1bb99a43b1f68068cdd9b
Copyright (c) 2020 phuoc-ng
License: MIT — https://1loc.dev/licenses/
Maintainer: Oveyon
Modifications: Curated Set-size variant; flat-array validation and SameValueZero note. Original MDX retained as inert source evidence.