---
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title: noDuplicateCase
description: Learn more about noDuplicateCase
---
import { Tabs, TabItem } from '@astrojs/starlight/components';

<Tabs>
<TabItem label="JavaScript (and super languages)" icon="seti:javascript">
## Summary
- Rule available since: `v1.0.0`
- Diagnostic Category: [`lint/suspicious/noDuplicateCase`](/reference/diagnostics#diagnostic-category)
- This rule is **recommended**, meaning it is enabled by default.
- This rule doesn't have a fix.
- The default severity of this rule is [**error**](/reference/diagnostics#error).
- Sources: 
  - Same as [`no-duplicate-case`](https://eslint.org/docs/latest/rules/no-duplicate-case)

## How to configure
```json title="biome.json"
{
	"linter": {
		"rules": {
			"suspicious": {
				"noDuplicateCase": "error"
			}
		}
	}
}

```
## Description
Disallow duplicate case labels.

If a switch statement has duplicate test expressions in case clauses, it is likely that a programmer copied a case clause but forgot to change the test expression.

## Examples

### Invalid

```js
switch (a) {
    case 1:
        break;
    case 1:
        break;
    default:
        break;
}
```

<pre class="language-text"><code class="language-text">code-block.js:4:10 <a href="https://biomejs.dev/linter/rules/no-duplicate-case">lint/suspicious/noDuplicateCase</a> ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━<br /><br />  <strong><span style="color: Tomato;">✖</span></strong> <span style="color: Tomato;">Duplicate case label.</span><br />  <br />    <strong>2 │ </strong>    case 1:<br />    <strong>3 │ </strong>        break;<br />  <strong><span style="color: Tomato;">&gt;</span></strong> <strong>4 │ </strong>    case 1:<br />   <strong>   │ </strong>         <strong><span style="color: Tomato;">^</span></strong><br />    <strong>5 │ </strong>        break;<br />    <strong>6 │ </strong>    default:<br />  <br />  <strong><span style="color: lightgreen;">ℹ</span></strong> <span style="color: lightgreen;">The first similar label is here:</span><br />  <br />    <strong>1 │ </strong>switch (a) &#123;<br />  <strong><span style="color: Tomato;">&gt;</span></strong> <strong>2 │ </strong>    case 1:<br />   <strong>   │ </strong>         <strong><span style="color: Tomato;">^</span></strong><br />    <strong>3 │ </strong>        break;<br />    <strong>4 │ </strong>    case 1:<br />  <br /></code></pre>

```js
switch (a) {
    case one:
        break;
    case one:
        break;
    default:
        break;
}
```

<pre class="language-text"><code class="language-text">code-block.js:4:10 <a href="https://biomejs.dev/linter/rules/no-duplicate-case">lint/suspicious/noDuplicateCase</a> ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━<br /><br />  <strong><span style="color: Tomato;">✖</span></strong> <span style="color: Tomato;">Duplicate case label.</span><br />  <br />    <strong>2 │ </strong>    case one:<br />    <strong>3 │ </strong>        break;<br />  <strong><span style="color: Tomato;">&gt;</span></strong> <strong>4 │ </strong>    case one:<br />   <strong>   │ </strong>         <strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><br />    <strong>5 │ </strong>        break;<br />    <strong>6 │ </strong>    default:<br />  <br />  <strong><span style="color: lightgreen;">ℹ</span></strong> <span style="color: lightgreen;">The first similar label is here:</span><br />  <br />    <strong>1 │ </strong>switch (a) &#123;<br />  <strong><span style="color: Tomato;">&gt;</span></strong> <strong>2 │ </strong>    case one:<br />   <strong>   │ </strong>         <strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><br />    <strong>3 │ </strong>        break;<br />    <strong>4 │ </strong>    case one:<br />  <br /></code></pre>

```js
switch (a) {
    case "1":
        break;
    case "1":
        break;
    default:
        break;
}
```

<pre class="language-text"><code class="language-text">code-block.js:4:10 <a href="https://biomejs.dev/linter/rules/no-duplicate-case">lint/suspicious/noDuplicateCase</a> ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━<br /><br />  <strong><span style="color: Tomato;">✖</span></strong> <span style="color: Tomato;">Duplicate case label.</span><br />  <br />    <strong>2 │ </strong>    case &quot;1&quot;:<br />    <strong>3 │ </strong>        break;<br />  <strong><span style="color: Tomato;">&gt;</span></strong> <strong>4 │ </strong>    case &quot;1&quot;:<br />   <strong>   │ </strong>         <strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><br />    <strong>5 │ </strong>        break;<br />    <strong>6 │ </strong>    default:<br />  <br />  <strong><span style="color: lightgreen;">ℹ</span></strong> <span style="color: lightgreen;">The first similar label is here:</span><br />  <br />    <strong>1 │ </strong>switch (a) &#123;<br />  <strong><span style="color: Tomato;">&gt;</span></strong> <strong>2 │ </strong>    case &quot;1&quot;:<br />   <strong>   │ </strong>         <strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><br />    <strong>3 │ </strong>        break;<br />    <strong>4 │ </strong>    case &quot;1&quot;:<br />  <br /></code></pre>

### Valid

```js
switch (a) {
    case 1:
        break;
    case 2:
        break;
    default:
        break;
}
```

```js
switch (a) {
    case one:
        break;
    case two:
        break;
    default:
        break;
}
```

```js
switch (a) {
    case "1":
        break;
    case "2":
        break;
    default:
        break;
}
```

## Related links

- [Disable a rule](/linter/#disable-a-rule)
- [Configure the code fix](/linter#configure-the-code-fix)
- [Rule options](/linter/#rule-options)
- [Source Code](https://github.com/biomejs/biome/blob/main/crates/biome_js_analyze/src/lint/suspicious/no_duplicate_case.rs)
- [Test Cases](https://github.com/biomejs/biome/blob/main/crates/biome_js_analyze/tests/specs/suspicious/noDuplicateCase)

</TabItem>
</Tabs>