Project built twice - how is this possible?

I hope I'm just doing something embarrassing.

1. git clone CPS --recursive
2. git checkout db101be19c2ce7df573386e0426d164abde427e9
3. msbuild /r /m /bl src\cps.sln /p:DebugType=embedded /p:NuGetInteractive=true /interactive

The binlog from the above build is located at `\\kirillo2\public\Bugs\MSBuild\5918\msbuild5918.binlog`

Open the binlog in the viewer and search for:
`$task CreatePackage project(Microsoft.VisualStudio.ProjectSystem.NuGet.csproj)`

I'm seeing the project Microsoft.VisualStudio.ProjectSystem.NuGet.csproj built twice. The second time CreatePackage runs it clashes because the output is already open by the first instance of the project running.

If everything I believe in is correct the project shouldn't build it's default targets again since it is already being built the first time. How are there two builds of this project? The timeline view confirms that they run concurrently:


The actual error I'm getting is:
```
C:\Users\kirill\.nuget\packages\nuget.build.packaging\0.2.2\build\NuGet.Build.Packaging.targets(377,3): The process cannot access the file 'C:\CPS\bin\Debug\packages\nuget\Microsoft.VisualStudio.ProjectSystem.16.8.368-pre-gdb101be19c.nupkg' because it is being used by another process. [C:\CPS\src\Microsoft.VisualStudio.ProjectSystem.NuGet\Microsoft.VisualStudio.ProjectSystem.NuGet.csproj]
```

I see the first time the project is built as a result of building all projects in the .sln. The second time it is built as a reference of Microsoft.VisualStudio.ProjectSystem.Sdk.NuGet.csproj

I'm building using 16.8.2:
```
16.8.2+25e4d540b
16.8.2.56705
```