Fix duplicate solution folder creation when adding multiple projects from same directory

src/Cli/dotnet/Commands/Solution/Add/SolutionAddCommand.cs

@@ -102,9 +102,17 @@ public override int Execute()
             relativeSolutionFolderPath = _solutionFolderPath;
         }
 
-        return string.IsNullOrEmpty(relativeSolutionFolderPath)
-            ? null
-            : solution.AddFolder(GetSolutionFolderPathWithForwardSlashes(relativeSolutionFolderPath));
+        if (string.IsNullOrEmpty(relativeSolutionFolderPath))
+        {
+            return null;
+        }
+
+        // Check if a solution folder with this path already exists
+        // Solution folder paths should be unique, so use SingleOrDefault
+        var solutionFolderPath = GetSolutionFolderPathWithForwardSlashes(relativeSolutionFolderPath);
+        var existingFolder = solution.SolutionFolders.SingleOrDefault(f => f.Path == solutionFolderPath);
+
+        return existingFolder ?? solution.AddFolder(solutionFolderPath);
     }
 
     private async Task AddProjectsToSolutionAsync(IEnumerable<string> projectPaths, CancellationToken cancellationToken)

test/dotnet.Tests/CommandTests/Solution/Add/GivenDotnetSlnAdd.cs

@@ -409,6 +409,46 @@ public void WhenDirectoryContainsMultipleProjectsItCancelsWholeOperation(string
                 .Should().BeVisuallyEquivalentTo(contentBefore);
         }
 
+        [Theory]
+        [InlineData("sln", ".sln")]
+        [InlineData("solution", ".sln")]
+        [InlineData("sln", ".slnx")]
+        [InlineData("solution", ".slnx")]
+        public async Task WhenMultipleProjectsFromSameDirectoryAreAddedSolutionFolderIsNotDuplicated(string solutionCommand, string solutionExtension)
+        {
+            var projectDirectory = _testAssetsManager
+                .CopyTestAsset("TestAppWithSlnAndCsprojFiles", identifier: $"GivenDotnetSlnAdd-{solutionCommand}{solutionExtension}")
+                .WithSource()
+                .Path;
+
+            var firstProject = Path.Combine("Multiple", "First.csproj");
+            var secondProject = Path.Combine("Multiple", "Second.csproj");
+            var cmd = new DotnetCommand(Log)
+                .WithWorkingDirectory(projectDirectory)
+                .Execute(solutionCommand, $"App{solutionExtension}", "add", firstProject, secondProject);
+            cmd.Should().Pass();
+
+            ISolutionSerializer serializer = SolutionSerializers.GetSerializerByMoniker(Path.Combine(projectDirectory, $"App{solutionExtension}"));
+            SolutionModel solution = await serializer.OpenAsync(Path.Combine(projectDirectory, $"App{solutionExtension}"), CancellationToken.None);
+
+            // The solution already has App project, plus we added First and Second = 3 total
+            var projectsInSolution = solution.SolutionProjects.ToList();
+            projectsInSolution.Count.Should().Be(3);
+            projectsInSolution.Should().Contain(p => p.FilePath.Contains("First.csproj"));
+            projectsInSolution.Should().Contain(p => p.FilePath.Contains("Second.csproj"));
+
+            // Should only have one solution folder for "Multiple", not two
+            var solutionFolders = solution.SolutionFolders.ToList();
+            solutionFolders.Count.Should().Be(1);
+            solutionFolders.Single().Path.Should().Contain("Multiple");
+
+            // Both new projects should be in the same solution folder
+            var solutionFolder = solutionFolders.Single();
+            var multipleProjects = projectsInSolution.Where(p => p.FilePath.Contains("Multiple")).ToList();
+            multipleProjects.Count.Should().Be(2);
+            multipleProjects.All(p => p.Parent?.Id == solutionFolder.Id).Should().BeTrue();
+        }
+
         [Theory]
         [InlineData("sln", ".sln")]
         [InlineData("solution", ".sln")]