People who are really interested in the subject will not complain. I for example have been programming for the past 15 years and I will still read something like an introduction to C programming, just because there might be some ideas, explainations or examples in it that will help me deepen the fundamentals of what I already know.
Arguably there are topics where this works better and topics where this works worse. In electronics or mechanics groking the fundamentals is so essential to the whole field that you should know them by hard. And the only way to know them by hard is spaced repetition. So having some overlap in the things you learn is not only necessary here, it is crucial.
I know you typed it twice, but I just never heard that expression.
I think/hope you mean "guarantee" rather than "sign", because, yes, someone who has mastered the material is indeed much more likely to pass the test than someone who has not. The existence of non-representative counterexamples does not refute that.
Being able to pass a test is indeed not a sign of mastery of the material (this might come as a shock to some! Just because you got good grades doesn’t mean you’ve mastered the subject! It just means you’re good at the test! But wait, why use tests then? Well, sit down, get comfortable and let me tell you a tale of how we got here…)
The fact this conversation exists highlights that our education system is silly. Without evidence, I subjectively like the ideas in “One World Schoolhouse” as an alternative.