On the one hand, as we know from the P vs. NP problem (at least if we assume the majority opinion), explaining a solution is much easier than coming up with it... and even easier if they copy from a good student who not only writes good code, but also documents it.

On the other hand, even if I am very confident that a student didn't write the code because they clearly don't understand it (which is often the case), this is difficult to uphold if the student appeals. For better or for worse, the greater accountability in grading and the availability of appeal processes means that you need to have some kind of objective evidence. "It was written in the rules that duplicate code would not be accepted, and this is clearly duplicate code" is objective. "I questioned both students and I found that this one couldn't correctly explain how the code works, so I'm sure he didn't write it" is not.

Note that I do this kind of questioning routinely (not only when cheating is involved) and take it into account in grades, because it of course makes sense to evaluate comprehension of the code... but outright failing a student on the grounds of an oral interview can easily get a professor into trouble.

> On the one hand, as we know from the P vs. NP problem (at least if we assume the majority opinion), explaining a solution is much easier than coming up with it... and even easier if they copy from a good student who not only writes good code, but also documents it.

You can ask “tricky” questions that someone who understands the material shouldn't have a problem answering, such as “if the problem required you to also do this, how would you change your code?”.

"I questioned both students and I found that this one couldn't correctly explain how the code works, so I'm sure he didn't write it" is not.

Fair enough. But at least you can give a bad grade for not understanding the course material.