Just to clarify, glass cockpits do have redundancy built into the systems, though Avidyn's seems to suffer a few drawbacks to theirs that are not present in the Garmin G-1000 series. One also needs to be cognizant of how a given package works in an individual aircraft. By that I mean the G-1000 in a Cessna cockpit has a different battery backup life capacity than the Diamond version. So, one should not be complacent about what they think they know about a given glass cockpit. It can change from manufacturer to manufacturer, and even model to model within the same manufacturer, based on what the manufacturer has designed in and desires.
In general terms though, the PFD (Primary Flight Display), shows an inset map replication of the MFD display normally on the G-1000 (yes, you can select this feature on or off), but if you should lose your PFD unit, it will automatically switch the functions of the PFD to the MFD instantly. So, that's one form of backup. Additionally, you have dual electrical systems, and a battery power reserve. If all that goes away, you still have your backup "steam gauges" (hope you have a flashlight handy at night as per the FAR's though!).
In addition to having backup capacity as illustrated above, systems like the Garmin G-1000 use FRU's (Field Replaceable Units), so if one component goes out, you simply "plug and play" a replacement into the system. Even the displays are FRU's, and work on a LAN (Local Area Network...just like a computer in your office) built into the aircraft. Currently, the PFD and MFD are actually identical pieces of equipment that can be swapped out fully interchangeably. Next year, however, Garmin will be including a number of new features, including a new alphanumeric flight-director style keypad, and the MFD will also have a fully integrated autopilot on it, so it will become a different part number than the PFD.