Do not throw when a status code with a codefix appears multiple times in the method body

Fixes https://github.com/aspnet/AspNetCore/issues/4480

src/Mvc/src/Microsoft.AspNetCore.Mvc.Api.Analyzers/AddResponseTypeAttributeCodeFixAction.cs

@@ -169,7 +169,12 @@ namespace Microsoft.AspNetCore.Mvc.Api.Analyzers
                 }
 
                 var statusCode = metadata.IsDefaultResponse ? 200 : metadata.StatusCode;
-                statusCodes.Add(statusCode, (statusCode, metadata.ReturnType));
+                if (!statusCodes.ContainsKey(statusCode))
+                {
+                    // If a status code appears multiple times in the actual metadata, pick the first one to
+                    // appear in the codefix
+                    statusCodes.Add(statusCode, (statusCode, metadata.ReturnType));
+                }
             }
 
             return statusCodes.Values;

src/Mvc/test/Mvc.Api.Analyzers.Test/AddResponseTypeAttributeCodeFixProviderIntegrationTest.cs

@@ -51,6 +51,9 @@ namespace Microsoft.AspNetCore.Mvc.Api.Analyzers
         [Fact]
         public Task CodeFixAddsStatusCodesFromObjectInitializer() => RunTest();
 
+        [Fact]
+        public Task CodeFixWorksWhenMultipleIdenticalStatusCodesAreInError() => RunTest();
+
         private async Task RunTest([CallerMemberName] string testMethod = "")
         {
             // Arrange

src/Mvc/test/Mvc.Api.Analyzers.Test/TestFiles/AddResponseTypeAttributeCodeFixProviderIntegrationTest/CodeFixWorksWhenMultipleIdenticalStatusCodesAreInError.Input.cs

@@ -0,0 +1,34 @@
+using System.Collections.Generic;
+using System.Linq;
+
+namespace Microsoft.AspNetCore.Mvc.Api.Analyzers._INPUT_
+{
+    [ApiController]
+    [Route("[controller]/[action]")]
+    public class CodeFixWorksWhenMultipleIdenticalStatusCodesAreInError : ControllerBase
+    {
+        public List<CodeFixWorksWhenMultipleIdenticalStatusCodesAreInErrorModel> Values { get; } = 
+            new List<CodeFixWorksWhenMultipleIdenticalStatusCodesAreInErrorModel>();
+
+        public ActionResult<CodeFixWorksWhenMultipleIdenticalStatusCodesAreInErrorModel> GetItem(int id)
+        {
+            if (id == 0)
+            {
+                return NotFound();
+            }
+
+            var model = Values.FirstOrDefault(m => m.Id == id);
+            if (model == null)
+            {
+                return NotFound();
+            }
+
+            return model;
+        }
+    }
+
+    public class CodeFixWorksWhenMultipleIdenticalStatusCodesAreInErrorModel
+    {
+        public int Id { get; set; }
+    }
+}

src/Mvc/test/Mvc.Api.Analyzers.Test/TestFiles/AddResponseTypeAttributeCodeFixProviderIntegrationTest/CodeFixWorksWhenMultipleIdenticalStatusCodesAreInError.Output.cs

@@ -0,0 +1,38 @@
+using System.Collections.Generic;
+using System.Linq;
+using Microsoft.AspNetCore.Http;
+
+namespace Microsoft.AspNetCore.Mvc.Api.Analyzers._OUTPUT_
+{
+    [ApiController]
+    [Route("[controller]/[action]")]
+    public class CodeFixWorksWhenMultipleIdenticalStatusCodesAreInError : ControllerBase
+    {
+        public List<CodeFixWorksWhenMultipleIdenticalStatusCodesAreInErrorModel> Values { get; } = 
+            new List<CodeFixWorksWhenMultipleIdenticalStatusCodesAreInErrorModel>();
+
+        [ProducesResponseType(StatusCodes.Status200OK)]
+        [ProducesResponseType(StatusCodes.Status404NotFound)]
+        [ProducesDefaultResponseType]
+        public ActionResult<CodeFixWorksWhenMultipleIdenticalStatusCodesAreInErrorModel> GetItem(int id)
+        {
+            if (id == 0)
+            {
+                return NotFound();
+            }
+
+            var model = Values.FirstOrDefault(m => m.Id == id);
+            if (model == null)
+            {
+                return NotFound();
+            }
+
+            return model;
+        }
+    }
+
+    public class CodeFixWorksWhenMultipleIdenticalStatusCodesAreInErrorModel
+    {
+        public int Id { get; set; }
+    }
+}