The context menu for any message or folder in the Message Levels Tab or Rule Groups Tab allows you to create a diagnostics display filter. You can select Show Only, so that only the diagnostics for that message or folder are displayed in the Diagnostics tab and opened file tabs, Hide so that the diagnostics for that message or folder are not displayed, or Show to redisplay diagnostics that are currently hidden. By a combination of these options, you could, for example, choose to show all instances of a single message for a particular component and Rule Group.
Right-click again and select Reset Filter on any item in the Message Levels Tab or Rule Groups Tab to remove the filters and redisplay all diagnostics.