Fix duplicated collection items when a constructor parameter name differs only by case from the property

src/libraries/Microsoft.Extensions.Configuration.Binder/src/ConfigurationBinder.cs

@@ -250,7 +250,7 @@ private static void BindProperties(object instance, IConfiguration configuration
                     continue;
                 }
 
-                if (constructorParameters is null || !constructorParameters.Any(p => p.Name == property.Name))
+                if (constructorParameters is null || !constructorParameters.Any(p => string.Equals(p.Name, property.Name, StringComparison.OrdinalIgnoreCase)))
                 {
                     BindProperty(property, instance, configuration, options);
                 }

src/libraries/Microsoft.Extensions.Configuration.Binder/tests/Common/ConfigurationBinderTests.Helpers.cs

@@ -21,6 +21,8 @@ public const bool NotSourceGenMode
             = true;
 #endif
 
+        public const bool SourceGenMode = !NotSourceGenMode;
+
         public static IConfiguration GetConfigurationFromJsonString(string json)
         {
             var builder = new ConfigurationBuilder();

src/libraries/Microsoft.Extensions.Configuration.Binder/tests/Common/ConfigurationBinderTests.TestClasses.cs

@@ -221,6 +221,30 @@ public string Value
 
         public record RecordWithArrayParameter(string[] Array);
 
+        public class GetterOnlyCollectionWithCaseMismatchedCtorParameter
+        {
+            public GetterOnlyCollectionWithCaseMismatchedCtorParameter(List<string> instances) => Instances = instances;
+            public List<string> Instances { get; }
+        }
+
+        public class SettableCollectionWithCaseMismatchedCtorParameter
+        {
+            public SettableCollectionWithCaseMismatchedCtorParameter(List<string> instances) => Instances = instances;
+            public List<string> Instances { get; set; }
+        }
+
+        public class GetterOnlyInterfaceCollectionWithCaseMismatchedCtorParameter
+        {
+            public GetterOnlyInterfaceCollectionWithCaseMismatchedCtorParameter(IList<string> instances) => Instances = instances;
+            public IList<string> Instances { get; }
+        }
+
+        public class ParamsCollectionCtor
+        {
+            public ParamsCollectionCtor(params List<string> instances) => Instances = instances;
+            public List<string> Instances { get; }
+        }
+
         public readonly record struct ReadonlyRecordStructTypeOptions(string Color, int Length);
 
         public class ContainerWithNestedImmutableObject

src/libraries/Microsoft.Extensions.Configuration.Binder/tests/Common/ConfigurationBinderTests.cs

@@ -1667,6 +1667,29 @@ public void CanBindOnParametersAndProperties_RecordWithArrayConstructorParameter
             Assert.Equal(new string[] { "a", "b", "c" }, options.Array);
         }
 
+        /// <summary>
+        /// When a constructor parameter name differs only by case from a matching collection property,
+        /// the binder must bind the collection once (through the constructor) and must not bind it again
+        /// through the property, which would otherwise duplicate the collection items.
+        /// </summary>
+        [Fact]
+        [ActiveIssue("https://github.com/dotnet/runtime/issues/83803", typeof(TestHelpers), nameof(TestHelpers.SourceGenMode))]
+        public void CanBindOnParametersAndProperties_GetterOnlyCollectionWithCaseMismatchedConstructorParameter()
+        {
+            string json = """
+            {
+                "Instances": [ "first", "second" ]
+            }
+            """;
+
+            IConfiguration config = TestHelpers.GetConfigurationFromJsonString(json);
+            string[] expected = new[] { "first", "second" };
+
+            Assert.Equal(expected, config.Get<GetterOnlyCollectionWithCaseMismatchedCtorParameter>().Instances);
+            Assert.Equal(expected, config.Get<SettableCollectionWithCaseMismatchedCtorParameter>().Instances);
+            Assert.Equal(expected, config.Get<GetterOnlyInterfaceCollectionWithCaseMismatchedCtorParameter>().Instances);
+            Assert.Equal(expected, config.Get<ParamsCollectionCtor>().Instances);
+        }
 
         public static IEnumerable<object[]> Configuration_TestData()
         {