I taught a course "in a previous life" and while I wasn't anything close to as strict as you say here, I can tell you the flip side: students would copy, modify

I taught a course "in a previous life" and while I wasn't anything close to as strict as you say here, I can tell you the flip side: students would copy, modify superficially (some, less superficially) and then claim "it's my work, we just talked, that's why the similarities!" (with some even having the nerve to say, "it's not copied, look, plagiarism detection software says similarity is less than x%!) Perhaps I was wrong but I really wanted the students who took the course to put in the work themselves. Just choose a different course/course set, you _knew_ this one was going to be hard! So yeah, the guideline was that we'll be eventual judges of what was "too similar" and if you're concerned, just don't discuss implementation details with anyone. I realize it prevents honest colaboration, and that's bad too... but sometimes it's a "damned if you do, damned if you don't" kind of situation.