# declaration-block-no-duplicate-custom-properties

Disallow duplicate custom properties within declaration blocks.

<!-- prettier-ignore -->
```css
a { --custom-property: pink; --custom-property: orange; }
/** ↑                        ↑
 * These duplicated custom properties */
```

This rule is case-sensitive.

The [`message` secondary option](../../../docs/user-guide/configure.md#message) can accept the arguments of this rule.

## Options

### `true`

The following patterns are considered problems:

<!-- prettier-ignore -->
```css
a { --custom-property: pink; --custom-property: orange; }
```

<!-- prettier-ignore -->
```css
a { --custom-property: pink; background: orange; --custom-property: orange }
```

The following patterns are _not_ considered problems:

<!-- prettier-ignore -->
```css
a { --custom-property: pink; }
```

<!-- prettier-ignore -->
```css
a { --custom-property: pink; --cUstOm-prOpErtY: orange; }
```

## Optional secondary options

### `ignoreProperties: ["/regex/", /regex/, "non-regex"]`

Ignore duplicates of specific properties.

Given:

```json
["--custom-property", "/ignored/"]
```

The following patterns are considered problems:

<!-- prettier-ignore -->
```css
a { --another-custom-property: 1; --another-custom-property: 1; }
```

The following patterns are _not_ considered problems:

<!-- prettier-ignore -->
```css
a { --custom-property: 1; --custom-property: 1; }
```

<!-- prettier-ignore -->
```css
a { --custom-ignored-property: 1; --custom-ignored-property: 1; }
```