As soon as I try to compile code that use C# 7 tuples with multi targets (netstandard2.0 and xamarin.iOS) I got the following error:
Predefined type 'System.ValueTuple`2' is declared in multiple referenced assemblies
I don't know which assemblies the conflict occurred in.
However, I realized I had not myself added the ValueTuple nuget package so I explicitly did and the error went away.