I agree with the accepted answer, but I think this is a case of very bad writing. The writer seems to be trying to say that the patients assume the practitioner has a duty that they may not actually have.
But that is very much not the standard use of the phrase "assumed duties." To assume, when the object is a duty or privilege, normally means "to take on along with something else."
- When you become a king/queen, you "assume the throne." The throne is something you get along with being king.
- When you become president, you "assume the duties of the presidency." The duties of the presidency are something you get by becoming presidency.
- When you buy a property with an unpaid lien, you "assume the debt." Because you got that property, you now have to pay the debt.
It seems like this writer had the phrase "assumed duties" bouncing around in their head and then misapplied it to refer to someone's inaccurate presumption about someone else's duties.