Does not support targeting multiple frameworks with conditions.

[dulsi]

Steps to reproduce

1. Create a project.
2. Add a package.
3. Modify the project to support multiple frameworks
   <TargetFrameworks>net6.0;net7.0;net8.0</TargetFrameworks>
4. Modify the project so that it uses different packages depending on target framework.

    <ItemGroup Condition="'$(TargetFramework)' == 'net8.0'">
        <PackageReference Include="Microsoft.Net.Http.Headers" Version="8.0.2" />
    </ItemGroup>

    <ItemGroup Condition="'$(TargetFramework)' == 'net7.0' Or '$(TargetFramework)' == 'net6.0'">
        <PackageReference Include="Microsoft.Net.Http.Headers" Version="2.2.8" />
    </ItemGroup>

Expected behavior
I think the best default would be to just check the latest framework. Probably add a command line argument to specify the framework.

Observed behavior
ArgumentException: An item with the same key has already been added. Key: Microsoft.Net.Http.Headers

[SteveDesmond-ca]

👍 same as #181: at the very least, it shouldn't crash!

I think, given the complexity, initially the app should just respond stating that multiple target frameworks aren't supported.