Hey, @sherif
I have contacted with Google team, they made me believe that it’s not a problem of firebase. The problem is with the System, Means the way you are using.
You have to find a different way of sending the messages. Because the way you are using this message feature is incorrect, if you send any message to any number then it arrived only for 1 time but if you try 2nd time then It doesn’t because the data ( means the number ) was saved in FB database so it will not send any message to it again.