var mock = Substitute.For<IMock>();
mock.Test.Returns(new StringInt());//this works
mock.Test.Returns(new IntString());//this fails

public interface IMock
{
    IMyInterface<string> Test { get; }
}

public interface IMyInterface<T>;

class IntString : IMyInterface<int>, IMyInterface<string>;

class StringInt : IMyInterface<string>, IMyInterface<int>;

foreach (var aCompatibleInstanceType in compatibleInstanceTypes)
{
	if (aCompatibleInstanceType.IsGenericType &&
		aCompatibleInstanceType.GetGenericTypeDefinition() == genericTypeDefinition)
	{
		// both are the same generic type. If their GenericTypeArguments match then they are equivalent
		return CallSpecification.TypesAreAllEquivalent(
			aCompatibleInstanceType.GenericTypeArguments, type.GenericTypeArguments);
	}
}