Don't add duplicate FrameworkReference

Rx.NET/Source/src/System.Reactive/build/System.Reactive.targets

@@ -7,6 +7,6 @@
     <Reference Condition="'$(UseWindowsRxVersion)' == 'true' "  Include="$(MSBuildThisFileDirectory)..\..\build\netcoreapp3.0\System.Reactive.dll" />
     <Reference Condition="'$(UseWindowsRxVersion)' != 'true' "  Include="$(MSBuildThisFileDirectory)..\..\lib\netstandard2.0\System.Reactive.dll" />
 
-    <FrameworkReference Condition="'$(UseWindowsRxVersion)' == 'true' " Include="Microsoft.WindowsDesktop.App" />
+    <FrameworkReference Include="Microsoft.WindowsDesktop.App" Condition="'$(UseWindowsRxVersion)' == 'true' and !@(FrameworkReference->AnyHaveMetadataValue('Identity', 'Microsoft.WindowsDesktop.App'))" />
   </ItemGroup>
-</Project>
+</Project>