While the "must" in the sentence "You must study hard" is used to tell you what you should be doing, the "must" in "You must be kidding" suggests that a statement is most likely true. Those two meanings of "must" are similar to the two common meanings of deber. While this fairly common Spanish verb usually is used to refer to an obligation of some sort, it also can be used to state that something is quite likely.

Comments
If I may share these (excerpted from Haugthon and Mifflin Spanish dictionary and from Concise Oxford Spanish dictionary:
deber as a noun can mean: 1)duty, 2)obligation, and 3) debt.
Also, deberes means homework
In its reflexive usage, deberse can mean 1) due to or caused by and 2) to have a personal bligation.
Interesting verb, I might say.
I hope this helps.
— A Student
A typo: obligation, NOT bligation. My mistake.
— A Student