I backup my files with Western Digital WD Passport 1.6TB external drive. . . .
My question is how long these units last? Does it pay to save on duplicate units or will the longevity defeat that procedure since two units could fail in the same time period?
The goal of any back-up strategy is to avoid a single point-of-failure. From your description, you have a primary datastore (your desktop computer) and a back-up device (the external drive). So to that extent, technically you're covered.
These days, even consumer-grade spinning disks are quite reliable. The mean-time-between-failure statistics provided by the manufacturers are usually accurate in my experience—but that's over a large population of drives, and individual drives may last much longer or fail much sooner.
My rule with respect to backing up important data (I designed and for a number of years managed the enterprise computing environment for a U.S. government agency) is that you need to be prepared for lightning to strike twice. So at the very least, I would recommend that you add a second external drive to independently back up the files on your desktop computer that currently are being backed up by the existing external drive.
Is that enough? Depends on how paranoid you are. I peg the needle on the back-up paranoia scale. I back up my files in essentially real-time from a second-floor office desktop computer to a file server located in the basement of my home and, redundantly and independently (i.e., using different software), to "cloud" storage provided by Amazon. All the files on the file server are backed up at hourly intervals to a second file server. Once a week, the files are archived from the primary file server to a five-drive network storage device which is designed to withstand a two-drive failure; each of these weekly archives is saved for three months before being replaced by a newer one. The most recent of these archives also is uploaded to "cloud" storage provided by Google. Currently, that last upload-to-the-cloud operation is manual; everything else is automated.
Another suggestion: regularly verify that your back-ups are good. Enterprise back-up software typically does this by reading each back-up operation after it has completed and comparing its contents with the original files, but as far as I know this is rarely if ever true of consumer back-up programs. A weekly random check of a few recently backed-up files will give you some confidence that the back-up software you're using is working properly.