A sound backup strategy always includes backups of your backups! Picture this: you've diligently backed up your computer's internal hard disk to an external drive. Then one day, lightning strikes and both drives are damagedor your home is robbed and all your equipment stolen. So much for your backup. Backup media can fail for all the same reasons your hard drive can fail. So having just one backup, in my opinion, is never enough. You should alternate between two or more sets of backup media for greater safety. If you've set up your backups to run on a schedule, this might mean using set A (a hard drive or a stack of CDs) every day for a week, then switching to set B (a different drive or stack of CDs) for each day of the following week, then switching backand so on.
So are two sets enough? It depends. Most experts recommend using at least three sets, of which one is always stored off-site. But this advice was first given in the days when the media commonly used for backups was much less reliable than what's available today. And the cost of three sets of mediaespecially hard drivescan be hard to swallow for the average home or small-business Mac user.
In my opinion, except for mission-critical business use, two sets each of duplicates and archives should be adequate for most users. If you back up to hard drives, this can mean two drives, each of which is partitioned to store both a duplicate and an archive. Of course, if you can afford a third set, your data will be somewhat saferand your backup routine will be somewhat easier. In any case, you certainly should keep one of those sets in another location all the time.