My coworker is absent 2-3 days every month due to vacation or sickness. My supervisor is making me her backup for the days she is out. I am not comfortable being her backup, since I do not know her position well enough. I have tried to learn her position but I can only do a part of it with confidence. Besides, I am overwhelmed with my current workload. I have expressed this to my supervisor but he still expects me to be her backup. He offered to do my work whenever I am backing her up, though I don't think that solves the problem. Can my supervisor have this expectation for me, and pass it off as "performing other duties as assigned?" Can I suggest to my supervisor that he backup my coworker whenever she's out?
Your supervisor is not passing it off as "performing other duties as assigned". It really is "performing other duties as assigned". Covering for absent co-workers is just the sort of situation for which those clauses are designed.
The problem that covering both jobs might be too much for one person is solved by your supervisor's willingness to cover your job. Your supervisor presumably has some reason for preferring that arrangement to himself covering for your co-worker. If your workload is excessive you should want your supervisor to cover for you so that the supervisor learns more about what is involved in your job.
That leaves the problem that you are not fully trained on your co-worker's job. You might suggest an occasional cross-training day during which you and your co-worker swap jobs. You would each learn how to do a better job of covering for the other. You could ask each other questions as needed.
If that person is absent, and their job absolutely needs to be done while they are absent, then someone has to do it. You ask "can my supervisor force me to do the job". If they couldn't, then you could force your supervisor to do the job, so your supervisor would be asking here as well :-)
So yes, your supervisor can force you to do the job, unless it is a job you can't do (like your colleague drives trucks and you don't have the license). Obviously a decent supervisor would take into account that you don't know the job well enough, so you might be slow and make mistakes, and that your own work doesn't get done.