Prevent adding a project multiple times to the same solution.

src/Main/Base/Test/ICSharpCode.SharpDevelop.Tests.csproj

@@ -97,6 +97,7 @@
     <Compile Include="ProcessRunner\ProcessExitedTestFixture.cs" />
     <Compile Include="ProcessRunner\ProcessRunnerNotStartedTestFixture.cs" />
     <Compile Include="ProcessRunner\StandardOutputFromProcessTestFixture.cs" />
+    <Compile Include="Project\AddExistingProjectTests.cs" />
     <Compile Include="Project\BeforeBuildCustomToolProjectItemsTests.cs" />
     <Compile Include="Project\MockSolution.cs" />
     <Compile Include="Project\ProjectCustomToolOptionsTests.cs" />

src/Main/Base/Test/Project/AddExistingProjectTests.cs

@@ -0,0 +1,76 @@
+// 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 ICSharpCode.Core;
+using ICSharpCode.SharpDevelop.Dom;
+using ICSharpCode.SharpDevelop.Workbench;
+using NUnit.Framework;
+using Rhino.Mocks;
+
+namespace ICSharpCode.SharpDevelop.Project
+{
+	public class AddExistingProjectTests : SDTestFixtureBase
+	{
+		public override void FixtureSetUp()
+		{
+			base.FixtureSetUp();
+			SD.Services.AddStrictMockService<IProjectService>();
+			SD.ProjectService.Stub(p => p.LoadProject(Arg<ProjectLoadInformation>.Is.Anything)).Do(new Func<ProjectLoadInformation, IProject>(LoadProject));
+		}
+		
+		[SetUp]
+		public void SetUp()
+		{
+			projectGuid = Guid.Parse("5AA08931-8802-42FE-A489-1A1CCF3B59E6");
+		}
+		
+		Guid projectGuid; // GUID of next project to be loaded
+		
+		IProject LoadProject(ProjectLoadInformation info)
+		{
+			var project = MockRepository.GenerateStrictMock<IProject>();
+			project.Stub(p => p.IdGuid).Return(projectGuid);
+			project.Stub(p => p.FileName).Return(info.FileName);
+			project.Stub(p => p.ParentSolution).Return(info.Solution);
+			project.Stub(p => p.ParentFolder).PropertyBehavior();
+			project.Stub(p => p.ProjectSections).Return(new SimpleModelCollection<SolutionSection>());
+			project.Stub(p => p.ConfigurationMapping).Return(new ConfigurationMapping());
+			project.Stub(p => p.IsStartable).Return(false);
+			project.Stub(p => p.ProjectLoaded()).Do(new Action(delegate { }));
+			return project;
+		}
+		
+		readonly FileName project1FileName = new FileName(@"c:\AddExistingProjectTests\Project1.csproj");
+		readonly FileName project2FileName = new FileName(@"c:\AddExistingProjectTests\Project2.csproj");
+		
+		ISolution CreateSolution()
+		{
+			var fileName = new FileName(@"c:\AddExistingProjectTests\AddExistingProjectTests.sln");
+			return new Solution(fileName, MockRepository.GenerateStub<IProjectChangeWatcher>(), MockRepository.GenerateStub<IFileService>());
+		}
+		
+		[Test]
+		public void AddingProjectUsesExistingGUID()
+		{
+			var solution = CreateSolution();
+			var project = solution.AddExistingProject(project1FileName);
+			Assert.AreEqual(projectGuid, project.IdGuid);
+		}
+		
+		[Test, ExpectedException(typeof(ProjectLoadException))]
+		public void AddDuplicateProject()
+		{
+			var solution = CreateSolution();
+			// add project1 twice:
+			solution.AddExistingProject(project1FileName);
+			solution.AddExistingProject(project1FileName);
+		}
+		
+//		[Test]
+//		public void AddTwoProjectsWithSameGUID()
+//		{
+//			
+//		}
+	}
+}

src/Main/SharpDevelop/Project/SolutionFolder.cs

@@ -131,6 +131,8 @@ namespace ICSharpCode.SharpDevelop.Project
 		
 		public IProject AddExistingProject(FileName fileName)
 		{
+			if (parentSolution.Projects.Any(p => p.FileName == fileName))
+				throw new ProjectLoadException("Project " + fileName + " is already part of this solution.");
 			ProjectLoadInformation loadInfo = new ProjectLoadInformation(parentSolution, fileName, fileName.GetFileNameWithoutExtension());
 			IProject project = SD.ProjectService.LoadProject(loadInfo);
 			Debug.Assert(project.IdGuid != Guid.Empty);