Deduplicate project properties before generation

Avoids build failures due to misconfiguration.

Closes #172

Author: kzu

src/ThisAssembly.Project/ThisAssembly.Project.targets

@@ -8,8 +8,11 @@
 
   <Target Name="InjectThisAssemblyProject" DependsOnTargets="$(InjectThisAssemblyProjectDependsOn)" 
           BeforeTargets="PrepareForBuild;CompileDesignTime;GenerateMSBuildEditorConfigFileShouldRun">
+    <ItemGroup>
+      <ProjectPropertyDistinct Include="@(ProjectProperty -> Distinct())" />
+    </ItemGroup>
     <PropertyGroup>
-      <ThisAssemblyProject>@(ProjectProperty, '|')</ThisAssemblyProject>
+      <ThisAssemblyProject>@(ProjectPropertyDistinct, '|')</ThisAssemblyProject>
     </PropertyGroup>
     <ItemGroup Condition="'$(ThisAssemblyProject)' != ''">
       <CompilerVisibleProperty Include="@(ProjectProperty)" />

src/ThisAssembly.Tests/ThisAssembly.Tests.csproj

@@ -49,6 +49,7 @@
   </PropertyGroup>
 
   <ItemGroup>
+    <ProjectProperty Include="Foo" />
     <ProjectProperty Include="Foo" />
     <Constant Include="Foo.Bar" Value="Baz" Comment="Yay!" />
     <Constant Include="Foo.Hello" Value="World" Comment="Comments make everything better 😍" />