Adding references that already exist for Xamarin android projects

Description

Paket adds transient references for xamarin that seem to be already added (possibly due to some xamarin project imports)

Not sure if this is actually pakets fault, but it does mean it doesn't work nice with xamarin.
I haven't tested if nuget fails similarily.

Repro steps

1. Create New solution
2. Add xamarin csharp android project
3. Add paket.references and paket.dependencies with "Hopac"
4. paket install
5. build in visual studio 2017

See. https://github.com/btrepp/xamarinpakethopac

Expected behavior

No build errors

Actual behavior

Severity	Code	Description	Project	File	Line	Suppression State
Error	CS1703	Multiple assemblies with equivalent identity have been imported: 'C:\Users\Beau\Code\xamarinpaket\packages\System.Xml.XDocument\lib\netstandard1.3\System.Xml.XDocument.dll' and 'C:\Program Files (x86)\Microsoft Visual Studio\2017\Community\Common7\IDE\ReferenceAssemblies\Microsoft\Framework\MonoAndroid\v1.0\Facades\System.Xml.XDocument.dll'. Remove one of the duplicate references.	xamarinpaket	C:\Users\Beau\Code\xamarinpaket\xamarinpaket\CSC	1	Active
Error	CS1703	Multiple assemblies with equivalent identity have been imported: 'C:\Users\Beau\Code\xamarinpaket\packages\System.Threading.Tasks.Parallel\lib\netstandard1.3\System.Threading.Tasks.Parallel.dll' and 'C:\Program Files (x86)\Microsoft Visual Studio\2017\Community\Common7\IDE\ReferenceAssemblies\Microsoft\Framework\MonoAndroid\v1.0\Facades\System.Threading.Tasks.Parallel.dll'. Remove one of the duplicate references.	xamarinpaket	C:\Users\Beau\Code\xamarinpaket\xamarinpaket\CSC	1	Active
Error	CS1703	Multiple assemblies with equivalent identity have been imported: 'C:\Users\Beau\Code\xamarinpaket\packages\System.Threading\lib\netstandard1.3\System.Threading.dll' and 'C:\Program Files (x86)\Microsoft Visual Studio\2017\Community\Common7\IDE\ReferenceAssemblies\Microsoft\Framework\MonoAndroid\v1.0\Facades\System.Threading.dll'. Remove one of the duplicate references.	xamarinpaket	C:\Users\Beau\Code\xamarinpaket\xamarinpaket\CSC	1	Active
Error	CS1703	Multiple assemblies with equivalent identity have been imported: 'C:\Users\Beau\Code\xamarinpaket\packages\System.Runtime.Numerics\lib\netstandard1.3\System.Runtime.Numerics.dll' and 'C:\Program Files (x86)\Microsoft Visual Studio\2017\Community\Common7\IDE\ReferenceAssemblies\Microsoft\Framework\MonoAndroid\v1.0\Facades\System.Runtime.Numerics.dll'. Remove one of the duplicate references.	xamarinpaket	C:\Users\Beau\Code\xamarinpaket\xamarinpaket\CSC	1	Active
Error	CS1703	Multiple assemblies with equivalent identity have been imported: 'C:\Users\Beau\Code\xamarinpaket\packages\System.Reflection.Emit.Lightweight\lib\netstandard1.3\System.Reflection.Emit.Lightweight.dll' and 'C:\Program Files (x86)\Microsoft Visual Studio\2017\Community\Common7\IDE\ReferenceAssemblies\Microsoft\Framework\MonoAndroid\v1.0\Facades\System.Reflection.Emit.Lightweight.dll'. Remove one of the duplicate references.	xamarinpaket	C:\Users\Beau\Code\xamarinpaket\xamarinpaket\CSC	1	Active
Error	CS1703	Multiple assemblies with equivalent identity have been imported: 'C:\Users\Beau\Code\xamarinpaket\packages\System.Reflection.Emit.ILGeneration\lib\netstandard1.3\System.Reflection.Emit.ILGeneration.dll' and 'C:\Program Files (x86)\Microsoft Visual Studio\2017\Community\Common7\IDE\ReferenceAssemblies\Microsoft\Framework\MonoAndroid\v1.0\Facades\System.Reflection.Emit.ILGeneration.dll'. Remove one of the duplicate references.	xamarinpaket	C:\Users\Beau\Code\xamarinpaket\xamarinpaket\CSC	1	Active
Error	CS1703	Multiple assemblies with equivalent identity have been imported: 'C:\Users\Beau\Code\xamarinpaket\packages\System.Reflection.Emit\lib\netstandard1.3\System.Reflection.Emit.dll' and 'C:\Program Files (x86)\Microsoft Visual Studio\2017\Community\Common7\IDE\ReferenceAssemblies\Microsoft\Framework\MonoAndroid\v1.0\Facades\System.Reflection.Emit.dll'. Remove one of the duplicate references.	xamarinpaket	C:\Users\Beau\Code\xamarinpaket\xamarinpaket\CSC	1	Active
Error	CS1703	Multiple assemblies with equivalent identity have been imported: 'C:\Users\Beau\Code\xamarinpaket\packages\System.ObjectModel\lib\netstandard1.3\System.ObjectModel.dll' and 'C:\Program Files (x86)\Microsoft Visual Studio\2017\Community\Common7\IDE\ReferenceAssemblies\Microsoft\Framework\MonoAndroid\v1.0\Facades\System.ObjectModel.dll'. Remove one of the duplicate references.	xamarinpaket	C:\Users\Beau\Code\xamarinpaket\xamarinpaket\CSC	1	Active
Error	CS1703	Multiple assemblies with equivalent identity have been imported: 'C:\Users\Beau\Code\xamarinpaket\packages\System.Linq.Queryable\lib\netstandard1.3\System.Linq.Queryable.dll' and 'C:\Program Files (x86)\Microsoft Visual Studio\2017\Community\Common7\IDE\ReferenceAssemblies\Microsoft\Framework\MonoAndroid\v1.0\Facades\System.Linq.Queryable.dll'. Remove one of the duplicate references.	xamarinpaket	C:\Users\Beau\Code\xamarinpaket\xamarinpaket\CSC	1	Active
Error	CS1703	Multiple assemblies with equivalent identity have been imported: 'C:\Users\Beau\Code\xamarinpaket\packages\System.Collections.Concurrent\lib\netstandard1.3\System.Collections.Concurrent.dll' and 'C:\Program Files (x86)\Microsoft Visual Studio\2017\Community\Common7\IDE\ReferenceAssemblies\Microsoft\Framework\MonoAndroid\v1.0\Facades\System.Collections.Concurrent.dll'. Remove one of the duplicate references.	xamarinpaket	C:\Users\Beau\Code\xamarinpaket\xamarinpaket\CSC	1	Active

Known workarounds

Removing the references added seems to result in a successful build, though paket adds them back on 'paket install'

Start by reproducing the issue from the linked xamarinpakethopac repository with paket install, then build the Xamarin Android project in Visual Studio 2017. Investigate how Paket adds the transient netstandard facade references; done means the project builds without CS1703 duplicate-reference errors after paket install.