Throw Error When Adding Projects with the Same Name but Different Paths to Solution Folder

src/Cli/dotnet/CliStrings.resx

@@ -252,6 +252,9 @@
   <data name="SolutionAlreadyContainsProject" xml:space="preserve">
     <value>Solution {0} already contains project {1}.</value>
   </data>
+  <data name="SolutionFolderAlreadyContainsProjectWithFilename" xml:space="preserve">
+    <value>Solution folder '{0}' already contains a project with the filename '{1}'.</value>
+  </data>
   <data name="SpecifyAtLeastOneReferenceToRemove" xml:space="preserve">
     <value>You must specify at least one reference to remove.</value>
   </data>

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

@@ -165,6 +165,37 @@ private void AddProject(SolutionModel solution, string fullProjectPath, ISolutio
         // Generate the solution folder path based on the project path
         SolutionFolderModel? solutionFolder = GenerateIntermediateSolutionFoldersForProjectPath(solution, solutionRelativeProjectPath);
 
+        // Check if a project with the same filename already exists in the solution folder
+        string projectFileName = Path.GetFileName(solutionRelativeProjectPath);
+        if (solutionFolder != null)
+        {
+            var rootFolder = solutionFolder;
+            while (rootFolder.Parent is SolutionFolderModel parentFolder)
+            {
+                rootFolder = parentFolder;
+            }
+
+            var existingProjectWithSameName = solution.SolutionProjects.FirstOrDefault(
+                p => IsInSameFolderHierarchy(p.Parent, rootFolder) && Path.GetFileName(p.FilePath).Equals(projectFileName, StringComparison.OrdinalIgnoreCase));
+
+            if (existingProjectWithSameName != null)
+            {
+                throw new GracefulException(string.Format(CliStrings.SolutionFolderAlreadyContainsProjectWithFilename, rootFolder.Name, projectFileName));
+            }
+        }
+
+        static bool IsInSameFolderHierarchy(SolutionItemModel? projectParent, SolutionFolderModel rootFolder)
+        {
+            var current = projectParent;
+            while (current != null)
+            {
+                if (current == rootFolder)
+                    return true;
+                current = current.Parent;
+            }
+            return false;
+        }
+
         SolutionProjectModel project;
 
         try

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

@@ -294,7 +294,7 @@ public void WhenNestedProjectIsAddedSolutionFoldersAreCreatedBuild(string soluti
 
         }
 
-        [Theory(Skip = "https://github.com/dotnet/sdk/issues/47859")]
+        [Theory]
         [InlineData("sln")]
         [InlineData("solution")]
         public void WhenNestedDuplicateProjectIsAddedToASolutionFolder(string solutionCommand)