Do not add an assembly reference from a PowerShell script if the reference already exists in the project.

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

@@ -61,8 +61,20 @@ namespace ICSharpCode.PackageManagement.EnvDTE
 		
 		internal void AddReference(string path)
 		{
-			var referenceItem = new ReferenceProjectItem(MSBuildProject, path);
-			projectService.AddProjectItem(MSBuildProject, referenceItem);
+			if (!HasReference(path)) {
+				var referenceItem = new ReferenceProjectItem(MSBuildProject, path);
+				projectService.AddProjectItem(MSBuildProject, referenceItem);
+			}
+		}
+		
+		bool HasReference(string include)
+		{
+			foreach (ReferenceProjectItem reference in GetReferences()) {
+				if (IsReferenceMatch(reference, include)) {
+					return true;
+				}
+			}
+			return false;
 		}
 		
 		internal IEnumerable<ProjectItem> GetReferences()
@@ -70,6 +82,11 @@ namespace ICSharpCode.PackageManagement.EnvDTE
 			return MSBuildProject.GetItemsOfType(ItemType.Reference);
 		}
 		
+		bool IsReferenceMatch(ReferenceProjectItem reference, string include)
+		{
+			return String.Equals(reference.Include, include, StringComparison.InvariantCultureIgnoreCase);
+		}
+		
 		internal void RemoveReference(ReferenceProjectItem referenceItem)
 		{
 			projectService.RemoveProjectItem(MSBuildProject, referenceItem);

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

@@ -70,6 +70,32 @@ namespace PackageManagement.Tests.EnvDTE
 			Assert.IsTrue(saved);
 		}
 		
+		[Test]
+		public void Add_AddGacAssemblyReferenceWhenReferenceAlreadyExistsInProject_ReferenceIsNotAddedToMSBuildProject()
+		{
+			CreateReferences();
+			msbuildProject.AddReference("System.Data");
+			
+			references.Add("System.Data");
+			
+			int count = msbuildProject.Items.Count;
+			
+			Assert.AreEqual(1, count);
+		}
+		
+		[Test]
+		public void Add_AddGacAssemblyReferenceWhenReferenceAlreadyExistsInProjectButWithDifferentCase_ReferenceIsNotAddedToMSBuildProject()
+		{
+			CreateReferences();
+			msbuildProject.AddReference("System.Data");
+			
+			references.Add("SYSTEM.DATA");
+			
+			int count = msbuildProject.Items.Count;
+			
+			Assert.AreEqual(1, count);
+		}
+		
 		[Test]
 		public void GetEnumerator_ProjectHasTwoReferences_TwoReferencesReturned()
 		{