Deduplicate Compile items in startup pipeline

Normalize Compile items with MSBuild Distinct() after selecting the active top-level startup file so duplicate Compile entries are removed before downstream targets run.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: kzu

src/SmallSharp/SmallSharp.targets

@@ -14,7 +14,7 @@
     <StartupFile Condition="'$(StartupFile)' == ''">$(S)</StartupFile>
     <StartupFile Condition="'$(StartupFile)' == ''">$(ActiveDebugProfile)</StartupFile>
     <FindStartupFile Condition="'$(StartupFile)' == '' or !Exists('$(StartupFile)')">true</FindStartupFile>
-    <StartupFileDependsOn>EnsureProperties;CollectStartupFile;ResolveStartupFile;SelectStartupFile;SelectTopLevelCompile;UpdateLaunchSettings;EmitTargets</StartupFileDependsOn>
+    <StartupFileDependsOn>EnsureProperties;CollectStartupFile;ResolveStartupFile;SelectStartupFile;SelectTopLevelCompile;DeduplicateCompile;UpdateLaunchSettings;EmitTargets</StartupFileDependsOn>
 
     <!-- For CLI dotnet run, users must set ImportProjectExtensionProps/ImportProjectExtensionTargets=true -->
     <SmallSharpPackagesProps>$(MSBuildProjectExtensionsPath)$(MSBuildProjectFile).smallsharp.props</SmallSharpPackagesProps>
@@ -172,6 +172,16 @@
     </ItemGroup>
   </Target>
 
+  <Target Name="DeduplicateCompile">
+    <ItemGroup>
+      <_DistinctCompile Remove="@(_DistinctCompile)" />
+      <_DistinctCompile Include="@(Compile -> Distinct())" />
+      <Compile Remove="@(Compile)" />
+      <Compile Include="@(_DistinctCompile)" />
+      <_DistinctCompile Remove="@(_DistinctCompile)" />
+    </ItemGroup>
+  </Target>
+
   <Target Name="AfterClean">
     <Delete Files="$(MSBuildProjectDirectory)\Properties\launchSettings.json"
             Condition="Exists('$(MSBuildProjectDirectory)\Properties\launchSettings.json')" />