Sry if I digress, Shai pls feel free to ammend my words
Cn1's push requires you to control this bit of logic server-side. In simple terms, keep a collection of notifications sent in a db somewhere and decide on the server whether new notifs deserve being pushed
Even if you inplemented your own push and not cn1's (not advisable), only on Android would you be able to prevent showing a notification after it has arrived. On iOS, you would still need to make the server choose sending an active or a silent notification