Added section "Duplicate FCM push notifications" (#1285)
Added section "Duplicate FCM push notifications" (#1285)

Extended document "FCM Tokens may change." by addressing the possible issue of end-users receiving duplicated FCM push notifications due to duplicated FCM registrations.
firebase/firebase-android-sdk,firebase/firebase-android-sdk,firebase/firebase-android-sdk,firebase/firebase-android-sdk,firebase/firebase-android-sdk,firebase/firebase-android-sdk,firebase/firebase-android-sdk,firebase/firebase-android-sdk