Implement "last feature flag wins" behavior for duplicate feature flag IDs

[Copilot]

This PR implements the "last feature flag wins" behavior when multiple feature flags have the same ID in the configuration, aligning with the Microsoft Feature Management schema used in .NET and JavaScript implementations.

Problem

Previously, when multiple feature flags had the same id in the configuration, the first one encountered would be used. This behavior was inconsistent with other Microsoft Feature Management implementations where the last matching feature flag should win.

Solution

Modified the feature flag resolution logic to:

1. Return the last matching feature flag instead of the first when multiple flags share the same ID
2. Return unique feature flag names in the list operation to avoid duplicates

Changes Made

Core Implementation

• _get_feature_flag(): Modified to iterate through all feature flags and return the last matching one
• _list_feature_flag_names(): Updated to return only unique feature flag names while preserving order

Example Behavior

feature_flags = {
    "feature_management": {
        "feature_flags": [
            {"id": "MyFlag", "enabled": "false"},
            {"id": "MyFlag", "enabled": "true"},   # This one wins
        ]
    }
}

feature_manager = FeatureManager(feature_flags)
assert feature_manager.is_enabled("MyFlag") == True  # Last flag wins
assert feature_manager.list_feature_flag_names() == ["MyFlag"]  # No duplicates

Testing

• Added comprehensive tests for duplicate feature flag scenarios
• Added tests for mixed unique/duplicate flag configurations
• Added corresponding async tests to ensure consistency
• All existing tests continue to pass (no regression)

References

• .NET implementation: Last one wins flag providers FeatureManagement-Dotnet#501
• JavaScript implementation: FeatureManagement-JavaScript

Fixes #52.

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[Copilot]

Pull Request Overview

Implements “last feature flag wins” resolution and deduplicated flag name listing to align with Microsoft Feature Management behavior.

• Core feature resolution now returns the last matching flag instead of the first.
• Flag name listing deduplicates IDs while preserving original order.
• Added sync and async tests covering duplicate-flag scenarios.

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
featuremanagement/_featuremanagerbase.py	Updated _get_feature_flag to reverse-search flags; dedupe names in _list_feature_flag_names.
tests/test_feature_manager.py	Added sync tests for duplicate-flag handling and mixed scenarios.
tests/test_feature_manager_async.py	Added async tests for duplicate-flag handling (last-wins behavior).

Comments suppressed due to low confidence (2)

tests/test_feature_manager.py:274

• [nitpick] If preserving order is important, you might assert exact list equality (flag_names == expected_names) instead of comparing sets to ensure the intended ordering is tested.

        assert set(flag_names) == set(expected_names)

tests/test_feature_manager_async.py:222

• Consider adding a test_duplicate_feature_flags_mixed_with_unique_async async test to mirror the sync mixed-flag scenario and ensure consistency across both suites.

    @pytest.mark.asyncio

[Copilot]

[nitpick] Consider replacing the manual index-based loop with Python’s reversed for better readability. For example:

for feature_flag in reversed(feature_flags):
    if feature_flag.get("id") == feature_flag_name:
        return FeatureFlag.convert_from_json(feature_flag)

Suggested change

	index = len(feature_flags) - 1
	while index >= 0:
	if feature_flags[index].get("id") == feature_flag_name:
	return FeatureFlag.convert_from_json(feature_flags[index])
	index -= 1
	for feature_flag in reversed(feature_flags):
	if feature_flag.get("id") == feature_flag_name:
	return FeatureFlag.convert_from_json(feature_flag)

[mrm9084]

I'm not sure if it maters given the data sizes of these. But going through backwards with the index will always be faster than calling reversed every time, then looking backwards. Thoughts?

@copilot

[mrm9084]

Looks good to me now, but needs another approval as this is basically my change.