Fix duplicate builder members when constructor parameter matches property

src/FluentBuilderGenerator/FileGenerators/FluentBuilderClassesGenerator.cs

@@ -436,10 +436,17 @@ private static (bool IsPrimaryConstructor, IReadOnlyList<IPropertyOrParameterSym
 
         foreach (var property in properties.Where(p => p.IsPublicSettable()).Select(p => new PropertyOrParameterSymbol(p, p.Type, p.IsInitOnly(), p.IsRequired)))
         {
-            if (propertiesPublicSettable.All(p => p.Name != property.Name))
+            var existingIndex = propertiesPublicSettable.FindIndex(p => string.Equals(p.Name, property.Name, StringComparison.OrdinalIgnoreCase));
+            if (existingIndex < 0)
             {
                 propertiesPublicSettable.Add(property);
             }
+            else if (propertiesPublicSettable[existingIndex].PropertyType == PropertyType.Parameter)
+            {
+                // Replace the constructor parameter entry with the property entry so that
+                // _isSet tracking and property-assignment logic are correctly generated.
+                propertiesPublicSettable[existingIndex] = property;
+            }
         }
 
         var propertiesPrivateSettable = accessibility != FluentBuilderAccessibility.PublicAndPrivate ? [] :
@@ -517,8 +524,7 @@ private static string GenerateSeveralMethods(FluentData fluentData, ClassSymbol
 
         if (isPrimaryConstructor)
         {
-            var parameters = propertiesPublicSettable
-                .Where(p => p.PropertyType == PropertyType.Parameter)
+            var parameters = publicConstructors[0].Parameters
                 .Select(p => $"_{p.Name.ToCamelCase()}.Value");
 
             output.AppendLine(20, $"else {{ instance = new {className}({string.Join(", ", parameters)}); }}");

tests/FluentBuilderGeneratorTests/DTO/ClassWithConstructorParamMatchingProperty.cs

@@ -0,0 +1,19 @@
+namespace FluentBuilderGeneratorTests.DTO;
+
+public enum StageType
+{
+    Build,
+    Test
+}
+
+public class ClassWithConstructorParamMatchingProperty
+{
+    public ClassWithConstructorParamMatchingProperty(StageType type)
+    {
+        Type = type;
+    }
+
+    public StageType Type { get; set; }
+
+    public string Name { get; set; } = string.Empty;
+}

tests/FluentBuilderGeneratorTests/FluentBuilderSourceGeneratorTests.cs

@@ -907,4 +907,42 @@ public void GenerateFiles_ForRecordWithPrimaryConstructor_Should_GenerateCorrect
 
         data.Should().BeEquivalentTo(new { Normal = "normal", Data = "t50" });
     }
+
+    [Fact]
+    public void GenerateFiles_ForClassWithConstructorParamMatchingProperty_ShouldNotDuplicate()
+    {
+        // Arrange (Issue #76: constructor parameter name matching a property name should not create duplicate members)
+        var path = "./DTO/ClassWithConstructorParamMatchingProperty.cs";
+        var sourceFile = new SourceFile
+        {
+            Path = path,
+            Text = File.ReadAllText(path),
+            AttributeToAddToClass = "FluentBuilder.AutoGenerateBuilder"
+        };
+
+        // Act
+        var result = _sut.Execute(Namespace, [sourceFile]);
+
+        // Assert
+        result.Valid.Should().BeTrue();
+        result.Files.Should().HaveCount(NumFiles);
+        result.Files.Should().NotContain(r => r.Path.EndsWith("Error.g.cs"));
+
+        var fileResult = result.Files[NumFiles - 1];
+        var filename = Path.GetFileName(fileResult.Path);
+
+        if (Write) File.WriteAllText($"../../../DTO/{filename}", fileResult.Text);
+
+        // Verify the generated code does not contain duplicate '_type' fields
+        var generatedCode = fileResult.Text;
+        var typeFieldCount = System.Text.RegularExpressions.Regex.Matches(generatedCode, @"private Lazy<.*> _type\b").Count;
+        typeFieldCount.Should().Be(1, "the '_type' field should only be declared once");
+
+        var withTypeMethodCount = System.Text.RegularExpressions.Regex.Matches(generatedCode, @"public .* WithType\(").Count;
+        withTypeMethodCount.Should().Be(2, "WithType should only have two overloads (value and Func)");
+
+        // Verify constructor/property wiring in generated code.
+        generatedCode.Should().Contain("else { instance = new ClassWithConstructorParamMatchingProperty(_type.Value); }");
+        generatedCode.Should().Contain("if (_typeIsSet) { Instance.Value.Type = _type.Value; }");
+    }
 }