noDuplicateMapKeys (YAML)

Language YAML

Summary

Rule available since: v2.6.0
Diagnostic Category: lint/nursery/noDuplicateMapKeys
This rule doesn’t have a fix.
The default severity of this rule is error.

How to configure

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{
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  "linter": {
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    "rules": {
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      "nursery": {
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        "noDuplicateMapKeys": "error"
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      }
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    }
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  }
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}

Description

Disallow two keys with the same name inside YAML maps.

The YAML specification requires mapping keys to be unique. When a key is repeated, the document becomes ambiguous: tooling disagree on which value wins, with some keeping the first entry, some the last, and others rejecting the document entirely. Duplicate keys are therefore usually a mistake, such as a copy-paste error or an unintended override.

Keys are compared by their resolved scalar value, so quoted and unquoted forms of the same key are treated as duplicates.

Examples

Invalid

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person:
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  name: John Doe
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  'name': Jane Doe
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  "name": John Smith

code-block.yaml:2:3 lint/nursery/noDuplicateMapKeys ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

  ✖ The key name was already declared.
  
    1 │ person:
  > 2 │   name: John Doe
      │   ^^^^
    3 │   'name': Jane Doe
    4 │   "name": John Smith
  
  ℹ This is where a duplicated key was declared again.
  
    1 │ person:
    2 │   name: John Doe
  > 3 │   'name': Jane Doe
      │   ^^^^^^
    4 │   "name": John Smith
    5 │ 
  
  ℹ This is where a duplicated key was declared again.
  
    2 │   name: John Doe
    3 │   'name': Jane Doe
  > 4 │   "name": John Smith
      │   ^^^^^^
    5 │ 
  
  ℹ Duplicate keys make the mapping ambiguous, and tools disagree on which value wins.
  
  ℹ Remove or rename the duplicate keys.
  
  ℹ This rule belongs to the nursery group, which means it is not yet stable and may change in the future. Visit https://biomejs.dev/linter/#nursery for more information.

1
person: { name: John Doe, name: Jane Doe }

code-block.yaml:1:11 lint/nursery/noDuplicateMapKeys ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

  ✖ The key name was already declared.
  
  > 1 │ person: { name: John Doe, name: Jane Doe }
      │           ^^^^
    2 │ 
  
  ℹ This is where a duplicated key was declared again.
  
  > 1 │ person: { name: John Doe, name: Jane Doe }
      │                           ^^^^
    2 │ 
  
  ℹ Duplicate keys make the mapping ambiguous, and tools disagree on which value wins.
  
  ℹ Remove or rename the duplicate keys.
  
  ℹ This rule belongs to the nursery group, which means it is not yet stable and may change in the future. Visit https://biomejs.dev/linter/#nursery for more information.

Valid

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person:
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  name: John Doe

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person: { name: John Doe }