Our use case of sending nudges to clients to check for new Echo messages suggests we should make these messages collapsible. Info on [1]:

A collapsible message is a message that may be replaced by a new message if it has yet to be delivered to the device. [...] Common use cases of collapsible messages are messages used to tell a mobile app to sync data from the server. [...] Only the most recent message is relevant.

We should set the appropriate parameter in message requests: collapseKey for messages to Android.

[1] https://firebase.google.com/docs/cloud-messaging/concept-options#collapsible_and_non-collapsible_messages