A computing system is described that receives notification data associated with a user, and responsive to determining that the user is interacting with a first computing device at a first particular time, outputs, for transmission to the first computing device, the notification data. The computing system determines that the user is interacting with a second computing device at a second particular time, and responsive to determining, based on the second particular time being less than a threshold amount of time after the first particular time, that the user did not view the notification data at the first computing device, outputs, for transmission to the second computing device, the notification data.