Do not offer inline hints for parameters that already named

src/EditorFeatures/Test2/InlineHints/CSharpInlineParameterNameHintsTests.vb

@@ -1298,5 +1298,95 @@ class A
 
             Await VerifyParamHints(input, output)
         End Function
+
+        <WpfFact, WorkItem("https://github.com/dotnet/roslyn/issues/59317")>
+        Public Async Function TestExistingNamedParameter1() As Task
+            Dim input =
+            <Workspace>
+                <Project Language="C#" CommonReferences="true">
+                    <Document>
+class C
+{
+    static void Main(string[] args)
+    {
+        Goo({|a:|}1, 2, b: 0);
+    }
+
+    static void Goo(int a, int b)
+    {
+
+    }
+}
+                    </Document>
+                </Project>
+            </Workspace>
+
+            Dim output =
+            <Workspace>
+                <Project Language="C#" CommonReferences="true">
+                    <Document>
+class C
+{
+    static void Main(string[] args)
+    {
+        Goo(a: 1, 2, b: 0);
+    }
+
+    static void Goo(int a, int b)
+    {
+
+    }
+}
+                    </Document>
+                </Project>
+            </Workspace>
+
+            Await VerifyParamHints(input, output)
+        End Function
+
+        <WpfFact, WorkItem("https://github.com/dotnet/roslyn/issues/59317")>
+        Public Async Function TestExistingNamedParameter2() As Task
+            Dim input =
+            <Workspace>
+                <Project Language="C#" CommonReferences="true">
+                    <Document>
+class C
+{
+    static void Main(string[] args)
+    {
+        Goo({|a:|}1, {|b:|}2, c: 0);
+    }
+
+    static void Goo(int a, int b)
+    {
+
+    }
+}
+                    </Document>
+                </Project>
+            </Workspace>
+
+            Dim output =
+            <Workspace>
+                <Project Language="C#" CommonReferences="true">
+                    <Document>
+class C
+{
+    static void Main(string[] args)
+    {
+        Goo(a: 1, b: 2, c: 0);
+    }
+
+    static void Goo(int a, int b)
+    {
+
+    }
+}
+                    </Document>
+                </Project>
+            </Workspace>
+
+            Await VerifyParamHints(input, output)
+        End Function
     End Class
 End Namespace

src/Features/CSharp/Portable/InlineHints/CSharpInlineParameterNameHintsService.cs

@@ -19,14 +19,10 @@ namespace Microsoft.CodeAnalysis.CSharp.InlineHints;
 /// as well as associate the adornments back to the parameter name
 /// </summary>
 [ExportLanguageService(typeof(IInlineParameterNameHintsService), LanguageNames.CSharp), Shared]
-internal class CSharpInlineParameterNameHintsService : AbstractInlineParameterNameHintsService
+[method: ImportingConstructor]
+[method: Obsolete(MefConstruction.ImportingConstructorMessage, error: true)]
+internal sealed class CSharpInlineParameterNameHintsService() : AbstractInlineParameterNameHintsService
 {
-    [ImportingConstructor]
-    [Obsolete(MefConstruction.ImportingConstructorMessage, error: true)]
-    public CSharpInlineParameterNameHintsService()
-    {
-    }
-
     protected override void AddAllParameterNameHintLocations(
          SemanticModel semanticModel,
          ISyntaxFactsService syntaxFacts,
@@ -66,12 +62,23 @@ private static void AddArguments(
         BaseArgumentListSyntax argumentList,
         CancellationToken cancellationToken)
     {
+        // Ensure we don't add an inline parameter name hint using the same name already present on another argument.
+        using var _ = PooledHashSet<string?>.GetInstance(out var presentNames);
+        foreach (var argument in argumentList.Arguments)
+        {
+            if (argument is { NameColon.Name.Identifier.ValueText: string nameText })
+                presentNames.Add(nameText);
+        }
+
         foreach (var argument in argumentList.Arguments)
         {
             if (argument.NameColon != null)
                 continue;
 
             var parameter = argument.DetermineParameter(semanticModel, cancellationToken: cancellationToken);
+            if (presentNames.Contains(parameter?.Name))
+                continue;
+
             buffer.Add((argument.Span.Start, argument, parameter, GetKind(argument.Expression)));
         }
     }