1. A computer-implemented method for preventing a user-activity event from triggering transmission of duplicative notifications to multiple applications that are installed on a client device, the computer-implemented method comprising:obtaining subscription data defining a plurality of subscriptions that correspond to the multiple applications, wherein individual subscriptions include trigger criteria defining user activity for triggering transmission of individual notifications to the client device in association with individual applications;
obtaining application priority data that indicates predefined user-activity event characteristics for prioritizing a first application over a second application for transmission of a notification;
receiving event data that indicates characteristics of the user-activity event, wherein the characteristics satisfy first trigger criteria of a first subscription that corresponds to the first application, and second trigger criteria of a second subscription that corresponds to the second application; and
based on the characteristics of the user-activity event corresponding to the predefined user-activity event characteristics:
fulfilling the first subscription to cause transmission of the notification to the client device in association with the first application, and
refraining from fulfilling the second subscription to prevent transmission of a duplicative notification to the client device in association with the second application.