Remove duplicates From the TargetFrameworks while doing restore.

[Anipik]

When we build a libraries project in VS, it tries to due a restore. In our new config system, we remove the oSGroup from tfms just before restore. So for projects with <TargetFrameworks>netcoreapp5.0-Windows_NT;netcoreapp5.0-Unix<TargetFrameworks>
ends with just

<TargetFrameworks>netcoreapp5.0;netcoreapp5.0</TargetFrameworks>

which is not a valid input for restore task. It wont effect the build in VS, but we should fix it becuase it would show as an error on the output pane in the vs and it would be required for enabling the restore on ref and srcs

The error looks like this

Error occurred while restoring NuGet packages: Invalid restore input. Duplicate frameworks found: 'netcoreapp5.0, netcoreapp5.0'. Input files: C:\git\runtime\src\libraries\System.IO.FileSystem\tests\System.IO.FileSystem.Tests.csproj.

cc @ViktorHofer @ericstj @safern @danmosemsft

[ViktorHofer]

@Anipik why is the Distinct operation not working which runs before Restore? https://github.com/dotnet/arcade/blob/f50767f96246063f33a6565d318f3adf9058bace/src/Microsoft.DotNet.Build.Tasks.TargetFramework.Sdk/src/build/Microsoft.DotNet.Build.Tasks.TargetFramework.Sdk.targets#L55

[Anipik]

If I remember is correctly, it was because these .Distinct only removes identical items and these items would have different originalSpec metadata(with the OSGroup). but I can verify again as well.

[Anipik]

you are right, its getting removed. I will debug the reason behind the failure

[ViktorHofer]

So the distinct operation actually works but there is another failure during restore?

[Anipik]

yes it seems that way. i checked the binlog and duplicates are getting removed.

[Anipik]

It fails only in the VS, restore is successful from the command line.

[ericstj]

Is this still happening?

[alexvy86]

It is still happening, I just ran into it. I see it when opening and trying to build System.Memory in VS.

[ericstj]

I believe Viktor made a fix for this but you need to make sure you have the latest VS preview and SDK. I know @safern hit this too and was able to get past it with an update.