Validate that ChoiceField choices are unique, and values submitted only once

[RealOrangeOne]

Code of Conduct

• I agree to follow Django's Code of Conduct

Feature Description

This suggestion is 2 very closely related suggestions, which likely could be fixed together.

• The choices argument to ChoiceField (and subclasses) should be validated as unique.
• If duplicate choices are submitted to a MultipleChoiceField (and subclasses), input should be either rejected or deduplicated.

Problem

When defining a ChoiceField (or subclass), its choices can contain duplicates:

MultipleChoiceField(choices=[("1", 1), ("1", 2)])

In this case, the first match in choices is used, which is at least deterministic.

Additionally, MultipleChoiceField allows duplication of submissions:

>>> f = MultipleChoiceField(choices=[("1", 1), ("2", 2)])

>>> f.clean(["1", "1"])
['1', '1']

This could be surprising, if a user expects values to already be validated as unique.

Request or proposal

proposal

Additional Details

Discovered as part of https://code.djangoproject.com/ticket/36913

Implementation Suggestions

Validating choices are unique should be a non-breaking change, since it would already not work as expected.

Requiring unique submissions (or stripping out duplicates) could be breaking if someone is depending on the functionality. It would be sad if the behaviour had to be opt-in, since there are performance benefits from assuming submissions are unique.

[github-actions]

Thank you RealOrangeOne for sharing your idea! We have a lot of them so please be patient. You can see the current queue here.

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[kishu279]

what i found is this -
Static Definition: Developers can define choices with duplicate keys, leading to ambiguous data selection (only the first match is reachable).
Runtime Submission: MultipleChoiceField does not validate that a user-submitted list contains unique values, potentially leading to data integrity issues in many-to-many relationships or database constraints.

Developer side solution
Logic: Iterate through self.choices.
Recursive Handling: If a choice entry is a list or tuple (nested group), recursively extract keys.
Validation: Use a set to track seen keys. If a duplicate is found, raise a SystemCheckError.

User-Facing: Form Validation at runtime
Logic: Before calling the existing validation (which checks if values are valid), check if they are unique.
Validation: ```python
if len(value) != len(set(value)):
raise ValidationError(self.error_messages['duplicate_values'], code='duplicate_values')

@RealOrangeOne check this

[LilyFirefly]

The Steering Council is moving this forward.