collapseKey property
String identifying a group of messages (for example, "Updates Available") that can be collapsed, so that only the last message gets sent when delivery can be resumed. This is used to avoid sending too many of the same messages when the device comes back online or becomes active.
There is no guarantee of the order in which messages get sent.
A maximum of four different collapse keys is allowed at any given time. This means FCM server can simultaneously store four different send-to-sync messages per client app. If you exceed this number, there is no guarantee which four collapse keys the FCM server will keep.
Default value: None
Implementation
final String? collapseKey;