nuits さんのコード (http://www.nuits.jp/entry/2016/09/26/221142) をベースに、画面遷移された初回だけ処理を実行させる

NotifyNavigationBehavior.cs

using System;
using System.Runtime.CompilerServices;
using Xamarin.Forms;

namespace Smallgeek.Behaviors
{
    public class NotifyNavigationBehavior : BindableBehavior<Page>
    {
        protected override void OnAttachedTo(Page bindable)
        {
            base.OnAttachedTo(bindable);
            bindable.Appearing += OnAppearing;
            bindable.Disappearing += OnDisappearing;
        }

        protected override void OnDetachingFrom(Page bindable)
        {
            bindable.Disappearing -= OnDisappearing;
            bindable.Appearing -= OnAppearing;
            base.OnDetachingFrom(bindable);
        }

        private void OnAppearing(object sender, EventArgs eventArgs)
        {
            (AssociatedObject.BindingContext as IAppearingAware)?.OnAppearing();

            var firstAppearing = AssociatedObject.BindingContext as IFirstAppearingAware;
            if (firstAppearing?.IsLoaded() == false)
            {
                firstAppearing.OnFirstAppearing();
                firstAppearing.MarkLoaded();
            }
        }

        private void OnDisappearing(object sender, EventArgs eventArgs)
        {
            (AssociatedObject.BindingContext as IDisappearingAware)?.OnDisappearing();
        }
    }

    public static class FirstAppearingAwareExtension
    {
        private readonly static ConditionalWeakTable<IFirstAppearingAware, Holder<bool>> holdPlaceState = new ConditionalWeakTable<IFirstAppearingAware, Holder<bool>>();

        public static bool IsLoaded(this IFirstAppearingAware self)
        {
            return holdPlaceState.GetOrCreateValue(self).Value;
        }

        public static void MarkLoaded(this IFirstAppearingAware self)
        {
            holdPlaceState.GetOrCreateValue(self).Value = true;
        }
    }

    class Holder<T> where T : struct
    {
        public T Value { get; set; }
    }
}