collapseKey property
An arbitrary string that identifies a group of messages that can be collapsed to ensure that only the last message is sent when delivery can resume. This helps avoid sending too many instances of the same messages when the recipient's device comes online again or becomes active.
Amazon Pinpoint specifies this value in the Firebase Cloud Messaging (FCM) collapse_key parameter when it sends the notification message to FCM.
Implementation
final String? collapseKey;