Why do some clients receive notifications twice, while others don't come at all?

Question

Push technologies

Hello!
I did send push notifications via firebase cloud messaging and ran into a few issues.
1) Sometimes 2 notifications come to the phone at once. Tested on different clients - all the same the logic is not clear.
2) Notifications do not come through mozilla. Although everything is fine in the request response: the status is success:1 and the Mozilla server is indicated in the "message_id":

{"multicast_id":6794913079640931029,"success":1,"failure":0,"canonical_ids":0,"results":[{"message_id":"https:\/\/updates.push.services.mozilla.com\/m\/bablabla"}]}