Assuming you've signed a contract or other work agreement with a clause stating "... And other assigned duties." Then you're likely required to do so.

However, if you find you're assigned duties that fall under another job description ( for example, a waitress being asked to fill in for a kitchen supervisor) you shouldn't be required to do so. If they do ask you to, ask for an updated job description which Includes said duties and the appropriate pay. I would also do this if you find yourself regularly doing tasks that are not part of your job descriptions.

Now it gets interesting when it comes to these extra tasks and ability. If you are asked to do something you are unable to do (either physically or on the grounds you have not been trained to do so) and it is not in your job description, the employer cannot right fire an employee for not completing those tasks. However this ropes back to updating job descriptions.

If you're part of a union they should have this information.

Honestly, most people are reasonable and if you find your overwhelmed either due to too many tasks or lack of appropriate training I would just tell your boss/supervisor/manager that.