Implement same behaviour as Visual Studio's EnvDTE.ProjectItems by throwing an exception when AddFileFromCopy called and the destination file exists.

src/AddIns/Misc/PackageManagement/Project/PackageManagement.csproj

@@ -94,6 +94,7 @@
     <Compile Include="Src\EnvDTE\Reference.cs" />
     <Compile Include="Src\EnvDTE\References.cs" />
     <Compile Include="Src\EnvDTE\Solution.cs" />
+    <Compile Include="Src\FileExistsException.cs" />
     <Compile Include="Src\IAddPackageReferenceView.cs" />
     <Compile Include="Src\IPackageActionRunner.cs" />
     <Compile Include="Src\IPackageFromRepository.cs" />

src/AddIns/Misc/PackageManagement/Project/Src/EnvDTE/ProjectItems.cs

@@ -27,9 +27,17 @@ namespace ICSharpCode.PackageManagement.EnvDTE
 			project.Save();
 		}
 		
+		void ThrowExceptionIfFileExists(string filePath)
+		{
+			if (fileService.FileExists(filePath)) {
+				throw new FileExistsException(filePath);
+			}
+		}
+		
 		void CopyFileIntoProject(string oldFileName, string fileName)
 		{
 			string newFileName = GetFileNameInProject(fileName);
+			ThrowExceptionIfFileExists(newFileName);
 			fileService.CopyFile(oldFileName, newFileName);
 		}
 		

src/AddIns/Misc/PackageManagement/Project/Src/FileExistsException.cs

@@ -0,0 +1,23 @@
+// Copyright (c) AlphaSierraPapa for the SharpDevelop Team (for details please see \doc\copyright.txt)
+// This code is distributed under the GNU LGPL (for details please see \doc\license.txt)
+
+using System;
+using System.IO;
+
+namespace ICSharpCode.PackageManagement
+{
+	public class FileExistsException : Exception
+	{
+		public FileExistsException(string fileName)
+			: base(GetErrorMessage(fileName))
+		{
+		}
+		
+		static string GetErrorMessage(string fileName)
+		{
+			fileName = Path.GetFileName(fileName);
+			string message = "Unable to add '{0}'. A file with that name already exists.";
+			return String.Format(message, fileName);
+		}
+	}
+}

src/AddIns/Misc/PackageManagement/Project/Src/IPackageManagementFileService.cs

@@ -11,5 +11,6 @@ namespace ICSharpCode.PackageManagement
 		void RemoveDirectory(string path);
 		void OpenFile(string fileName);
 		void CopyFile(string oldFileName, string newFileName);
+		bool FileExists(string fileName);
 	}
 }

src/AddIns/Misc/PackageManagement/Project/Src/PackageManagementFileService.cs

@@ -2,6 +2,7 @@
 // This code is distributed under the GNU LGPL (for details please see \doc\license.txt)
 
 using System;
+using System.IO;
 using ICSharpCode.SharpDevelop;
 using ICSharpCode.SharpDevelop.Gui;
 
@@ -48,5 +49,10 @@ namespace ICSharpCode.PackageManagement
 				FileService.CopyFile(oldFileName, newFileName, isDirectory: false, overwrite: false);
 			}
 		}
+		
+		public bool FileExists(string fileName)
+		{
+			return File.Exists(fileName);
+		}
 	}
 }

src/AddIns/Misc/PackageManagement/Test/Src/EnvDTE/ProjectItemsTests.cs

@@ -4,11 +4,13 @@
 using System;
 using System.Collections;
 using System.Collections.Generic;
+
+using ICSharpCode.PackageManagement;
 using ICSharpCode.PackageManagement.EnvDTE;
-using DTE = ICSharpCode.PackageManagement.EnvDTE;
 using ICSharpCode.SharpDevelop.Project;
 using NUnit.Framework;
 using PackageManagement.Tests.Helpers;
+using DTE = ICSharpCode.PackageManagement.EnvDTE;
 
 namespace PackageManagement.Tests.EnvDTE
 {
@@ -306,5 +308,22 @@ namespace PackageManagement.Tests.EnvDTE
 			
 			Assert.AreEqual("Program.cs", projectItemName);
 		}
+		
+		[Test]
+		public void AddFileFromCopy_FileAlreadyExistsOnFileSystem_ThrowsException()
+		{
+			CreateProjectItems();
+			msbuildProject.FileName = @"d:\projects\myproject\myproject\myproject.csproj";
+			string fileName = @"d:\projects\myproject\packages\tools\test.cs";
+			
+			string existingFile = @"d:\projects\myproject\myproject\test.cs";
+			fakeFileService.ExistingFileNames.Add(existingFile);
+			
+			Exception ex = 
+				Assert.Throws(typeof(FileExistsException), () => projectItems.AddFromFileCopy(fileName));
+			
+			bool contains = ex.Message.Contains("'test.cs'");
+			Assert.IsTrue(contains);
+		}
 	}
 }

src/AddIns/Misc/PackageManagement/Test/Src/Helpers/FakeFileService.cs

@@ -2,6 +2,7 @@
 // This code is distributed under the GNU LGPL (for details please see \doc\license.txt)
 
 using System;
+using System.Collections.Generic;
 using ICSharpCode.PackageManagement;
 using ICSharpCode.SharpDevelop.Project;
 
@@ -54,5 +55,12 @@ namespace PackageManagement.Tests.Helpers
 			OldFileNamePassedToCopyFile = oldFileName;
 			NewFileNamePassedToCopyFile = newFileName;
 		}
+		
+		public List<string> ExistingFileNames = new List<string>();
+		
+		public bool FileExists(string fileName)
+		{
+			return ExistingFileNames.Contains(fileName);
+		}
 	}
 }