Just found in a Normal Mode playthrough - if clicking the notification for completing a contract while the notification is expiring from being on the screen for long enough, you get the money for completing it with the notification disappears and again when clicking the notification closed. Similar to Spiffing’s exploit in Pirate Mode, but not quite the same in that I’ve only seen it duplicate the money once.
After further testing, I’ve found this consistently happens if you right click then immediately left click the notification as well.