Paket 5.x references NETStandard.Library twice

Description

While testing it I tried: Paket 5.1.0, 5.1.3, 5.1.4, 5.1.5 and 5.1.6

When using Paket 5.x it references NETStandard.Library twice (kind of).

Repro steps

mkdir solution && cd solution && dotnet new sln
curl -L -o paket.cmd http://bit.ly/2ooaInt
paket init
echo framework: netcoreapp1.1 >> paket.dependencies
echo nuget Grpc >> paket.dependencies
mkdir project && cd project && dotnet new classlib --framework netstandard1.6 && cd ..
dotnet sln solution.sln add project\project.csproj
echo Grpc > project\paket.references
paket install && dotnet restore

Now open this solution with Visual Studio, and:

• Unfold "Dependencies", "Nuget" and "SDK". "NETStandard.Library" is in both, but in "Nuget" has yellow warning triangle;
• All dependencies of NETStandard.Library as also referenced twice (from "Nuget" and "SDK");
• Visual Studio show warning on "Error List" tab: A PackageReference for 'NETStandard.Library' was included in your project. This package is implicitly referenced by the .NET SDK and you do not typically need to reference it from your project. For more information, see https://aka.ms/sdkimplicitrefs

Expected behavior

• Only "Grpc" in "Nuget" dependencies ("NETStandard.Library" should be in SDK). This is also how Paket 4.8.8 behaves.

Actual behavior

Dependecies are referenced twice.

Known workarounds

Use Paket 4.8.8.

Start with the listed Paket 5.x reproduction commands and compare the generated dependencies with the Paket 4.8.8 workaround. Check how the project handles the generated paket references and the .NET SDK's implicit NETStandard.Library reference. Done means Grpc remains under NuGet while NETStandard.Library and its dependencies appear only under SDK, without the Visual Studio warning.