[release/10.0.3xx] Fix dotnet tool runner stripping duplicate flags when rollForward is enabled (#53821)

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: marcpopMSFT <12663534+marcpopMSFT@users.noreply.github.com>
Co-authored-by: Marc Paine <marcpop@microsoft.com>
Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

Author: 4 people

src/Cli/dotnet/CommandFactory/CommandResolution/MuxerCommandSpecMaker.cs

@@ -38,7 +38,7 @@ internal static CommandSpec CreatePackageCommandSpecUsingMuxer(
         {
             if (rollForwardArgument.Any())
             {
-                arguments.AddRange(commandArguments.Except(rollForwardArgument));
+                arguments.AddRange(commandArguments.Where(arg => !arg.Equals("--allow-roll-forward", StringComparison.OrdinalIgnoreCase)));
             }
             else
             {

test/dotnet.Tests/CommandTests/Tool/Run/ToolRunCommandTests.cs

@@ -43,6 +43,24 @@ public void WhenRunWithRollForwardOptionItShouldIncludeRollForwardInNativeHost()
             result.Args.Should().ContainAll("--roll-forward", "Major", fakeExecutable.Value);
         }
 
+        [Fact]
+        public void WhenRunWithRollForwardOptionItShouldPreserveDuplicateArgumentsForwardedToTool()
+        {
+            (FilePath fakeExecutable, LocalToolsCommandResolver localToolsCommandResolver) = DefaultSetup("a");
+            IEnumerable<string> testForwardArgument = ["--var", "a", "--var", "b"];
+
+            var result = localToolsCommandResolver.ResolveStrict(new CommandResolverArguments()
+            {
+                CommandName = "dotnet-a",
+                CommandArguments = testForwardArgument
+            }, allowRollForward: true);
+
+            result.Should().NotBeNull();
+            result.Args.Should().ContainAll("--roll-forward", "Major", fakeExecutable.Value);
+            // Verify duplicate forwarded arguments are preserved in order.
+            result.Args.Should().Contain("--var a --var b");
+        }
+
         [Fact]
         public void WhenRunWithoutRollForwardOptionItShouldNotIncludeRollForwardInNativeHost()
         {