Dedupe when returning all feature names

src/Microsoft.FeatureManagement/FeatureManager.cs

@@ -196,11 +196,17 @@ public IAsyncEnumerable<string> GetFeatureNamesAsync()
         /// <returns>An enumerator which provides asynchronous iteration over the feature names registered in the feature manager.</returns>
         public async IAsyncEnumerable<string> GetFeatureNamesAsync([EnumeratorCancellation] CancellationToken cancellationToken = default)
         {
+            var featureNamesReturned = new HashSet<string>();
             await foreach (FeatureDefinition featureDefinition in _featureDefinitionProvider.GetAllFeatureDefinitionsAsync().ConfigureAwait(false))
             {
                 cancellationToken.ThrowIfCancellationRequested();
 
-                yield return featureDefinition.Name;
+                if (!featureNamesReturned.Contains(featureDefinition.Name))
+                {
+                    yield return featureDefinition.Name;
+
+                    featureNamesReturned.Add(featureDefinition.Name);
+                }
             }
         }
 

tests/Tests.FeatureManagement/FeatureManagementTest.cs

@@ -394,6 +394,79 @@ public async Task LastFeatureFlagWins()
 
             Assert.True(await featureManager.IsEnabledAsync(Features.DuplicateFlag));
         }
+
+        [Fact]
+        public async Task DedupesFeatureNames()
+        {
+            IConfiguration configuration = new ConfigurationBuilder()
+                .AddInMemoryCollection(new Dictionary<string, string>
+                {
+                    { $"FeatureManagement:{Features.DuplicateFlag}", "False" }
+                })
+                .AddInMemoryCollection(new Dictionary<string, string>
+                {
+                    { $"FeatureManagement:{Features.DuplicateFlag}", "True" }
+                })
+                .Build();
+
+            IServiceCollection services = new ServiceCollection();
+
+            services
+                .AddSingleton(configuration)
+                .AddFeatureManagement();
+
+            ServiceProvider serviceProvider = services.BuildServiceProvider();
+
+            IFeatureManager featureManager = serviceProvider.GetRequiredService<IFeatureManager>();
+
+            var featureNames = await featureManager.GetFeatureNamesAsync().ToListAsync();
+            Assert.Equal(featureNames.Distinct().Count(), featureNames.Count);
+            Assert.True(await featureManager.IsEnabledAsync(Features.DuplicateFlag));
+        }
+    }
+
+    public class FeatureManagementMultipleSchemasTest
+    {
+        [Fact]
+        public async Task DedupesFeatureNames()
+        {
+            IConfiguration configuration = new ConfigurationBuilder()
+            .AddJsonFile("appsettings.MultipleSchemas.json")
+            .Build();
+
+            IServiceCollection services = new ServiceCollection();
+
+            services
+                .AddSingleton(configuration)
+                .AddFeatureManagement();
+
+            ServiceProvider serviceProvider = services.BuildServiceProvider();
+
+            IFeatureManager featureManager = serviceProvider.GetRequiredService<IFeatureManager>();
+
+            var featureNames = await featureManager.GetFeatureNamesAsync().ToListAsync();
+            Assert.Equal(featureNames.Distinct().Count(), featureNames.Count);
+        }
+
+        [Fact]
+        public async Task PrioritizesMicrosoftSchemaConfiguration()
+        {
+            IConfiguration configuration = new ConfigurationBuilder()
+                .AddJsonFile("appsettings.MultipleSchemas.json")
+                .Build();
+
+            IServiceCollection services = new ServiceCollection();
+
+            services
+                .AddSingleton(configuration)
+                .AddFeatureManagement();
+
+            ServiceProvider serviceProvider = services.BuildServiceProvider();
+
+            IFeatureManager featureManager = serviceProvider.GetRequiredService<IFeatureManager>();
+
+            Assert.True(await featureManager.IsEnabledAsync(Features.DuplicateFlag));
+        }
     }
 
     public class FeatureManagementFeatureFilterGeneralTest

tests/Tests.FeatureManagement/Tests.FeatureManagement.csproj

@@ -1,4 +1,4 @@
-<Project Sdk="Microsoft.NET.Sdk">
+<Project Sdk="Microsoft.NET.Sdk">
 
   <PropertyGroup>
     <TargetFrameworks>net48;net8.0;net9.0</TargetFrameworks>
@@ -43,6 +43,9 @@
     <None Update="appsettings.json">
       <CopyToOutputDirectory>Always</CopyToOutputDirectory>
     </None>
+    <None Update="appsettings.MultipleSchemas.json">
+      <CopyToOutputDirectory>Always</CopyToOutputDirectory>
+    </None>
     <None Update="DotnetFeatureManagementSchema.json">
       <CopyToOutputDirectory>Always</CopyToOutputDirectory>
     </None>

tests/Tests.FeatureManagement/appsettings.MultipleSchemas.json

@@ -0,0 +1,22 @@
+{
+  "Logging": {
+    "LogLevel": {
+      "Default": "Warning"
+    }
+  },
+  "AllowedHosts": "*",
+
+  // MS schema will be prioritized, feature should be enabled
+  "feature_management": {
+    "feature_flags": [
+      {
+        "id": "DuplicateFlag",
+        "enabled": true
+      }
+    ]
+  },
+
+  "FeatureManagement": {
+    "DuplicateFlag": false
+  }
+}