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avoid-duplicate-collection-elements

added in: 1.13.0
warning
⚙️
Pro+

Warns when a collection has duplicate collection elements.

Duplicate collection elements are usually the result of a typo and a sign of a bug.

⚙️ Config​

Set ignore-literals (default is false) to exclude literals (example).

analysis_options.yaml
dart_code_metrics:
  rules:
    - avoid-duplicate-collection-elements:
        ignore-literals: false

Example​

❌ Bad:

void fn() {
  final list = <String>[...];

  final anotherList = [
    ...list,
    ...list, // LINT: Avoid duplicate collection elements. Try removing the duplicate element or replacing it with a different element.
    if (list.isNotEmpty) 'value',
    if (list.isNotEmpty) 'value', // LINT: Avoid duplicate collection elements. Try removing the duplicate element or replacing it with a different element.
  ];

  final map = {
    ...{
      'key': 'value', // LINT: Avoid duplicate collection elements. Try removing the duplicate element or replacing it with a different element.
    },
    ...{
      'key': 'value', // LINT: Avoid duplicate collection elements. Try removing the duplicate element or replacing it with a different element.
    },
  };
}

✅ Good:

void fn() {
  final list = <String>[...];

  final anotherList = [
    ...list,
    if (list.isNotEmpty) 'value', // Correct, no duplicates
  ];

  final map = {
    ...{
      'key': 'value',
    },
  };
}

Example with "ignore-literals"​

Config
analysis_options.yaml
dart_code_metrics:
  rules:
    - avoid-duplicate-collection-elements:
        ignore-literals: true

✅ Good:

void fn() {
  final list = [
    'value',
    'value', // Correct, ignored string literal
  ];
}

Additional Resources​