Describe the bug

When using Scriban with PackageScribanIncludeSource=true in a netstandard2.0 source generator project, builds fail with multiple duplicate type/attribute errors when the project also references the Polyfill NuGet package.

The errors are caused by both Scriban and Polyfill providing implementations of trimming/AOT-related attributes such as:

• DynamicallyAccessedMembersAttribute
• RequiresDynamicCodeAttribute
• RequiresUnreferencedCodeAttribute
• UnconditionalSuppressMessageAttribute
• DynamicallyAccessedMemberTypes

This results in compiler errors like:

• CS0101: namespace already contains a definition
• CS0579: duplicate attribute
• CS0121 / CS0229: ambiguous calls

To Reproduce

Project setup:

• Target framework: netstandard2.0
• Project type: Roslyn source generator
• Scriban: 7.0.5
• Polyfill: 9.23.0

Relevant csproj configuration:

<PropertyGroup>
  <TargetFramework>netstandard2.0</TargetFramework>
  <PackageScribanIncludeSource>true</PackageScribanIncludeSource>
</PropertyGroup>

<ItemGroup>
  <PackageReference Include="Scriban" Version="7.0.5" IncludeAssets="build" PrivateAssets="all" />
  <PackageReference Include="Polyfill" Version="9.23.0">
    <PrivateAssets>all</PrivateAssets>
    <IncludeAssets>runtime; build; native; contentfiles; analyzers; buildtransitive</IncludeAssets>
  </PackageReference>
</ItemGroup>

Result:
Build fails with dozens of duplicate type/attribute errors due to overlapping definitions between:

• Scriban source-included file: AotAttributes.cs
• Polyfill content files under Trimming/*.cs

Expected behavior

Scriban source include should coexist with common polyfill packages without requiring manual exclusion of files.

Actual behavior

Compilation fails due to duplicate definitions of trimming/AOT attributes.

Workaround

Manually excluding the overlapping Polyfill files resolves the issue:

<ItemGroup>
  <Compile Remove="$(NuGetPackageRoot)polyfill\9.23.0\contentFiles\cs\netstandard2.0\Trimming\DynamicallyAccessedMembersAttribute.cs" />
  <Compile Remove="$(NuGetPackageRoot)polyfill\9.23.0\contentFiles\cs\netstandard2.0\Trimming\DynamicallyAccessedMemberTypes.cs" />
  <Compile Remove="$(NuGetPackageRoot)polyfill\9.23.0\contentFiles\cs\netstandard2.0\Trimming\RequiresDynamicCodeAttribute.cs" />
  <Compile Remove="$(NuGetPackageRoot)polyfill\9.23.0\contentFiles\cs\netstandard2.0\Trimming\RequiresUnreferencedCodeAttribute.cs" />
  <Compile Remove="$(NuGetPackageRoot)polyfill\9.23.0\contentFiles\cs\netstandard2.0\Trimming\UnconditionalSuppressMessageAttribute.cs" />
</ItemGroup>

Additional context

This scenario is likely to affect source generator authors targeting netstandard2.0, where:

• Source inclusion (PackageScribanIncludeSource=true) is preferred to avoid dependency leakage
• Polyfill packages are commonly used to provide missing BCL features

Because both Scriban and Polyfill independently include the same AOT/trimming attributes, they are not composable without manual intervention.

Suggested improvement (optional)

Consider one of the following:

• Provide a compile-time switch (e.g., SCRIBAN_NO_AOT_ATTRIBUTES) to disable Scriban’s attribute definitions
• Conditionally include attributes only if they are not already defined
• Document this interaction with Polyfill and similar packages