Missing tests for duplicate students in grade-school exercise

[marcelweikum]

Hey! This is my first issue and potentially my first PR (if I’m allowed to contribute 😄).

In the instructions for the grade-school exercise, it is stated that:

cpp/exercises/practice/grade-school/.docs/instructions.md

Lines 20 to 21 in 32fe0da

	Note that all our students only have one name (It's a small town, what do you want?) and each student cannot be added more than once to a grade or the roster.
	In fact, when a test attempts to add the same student more than once, your implementation should indicate that this is incorrect.

"In fact, when a test attempts to add the same student more than once, your implementation should indicate that this is incorrect."

However, there are currently no tests that enforce this behavior.

I see two options:

1. Remove the instruction about duplicates, since it’s not being tested.
2. Add test cases to enforce the behavior, such as:
   Preventing the same student from being added multiple times to the same grade
   Preventing the same student from being added multiple times to the entire roster (if that’s the intended rule)
   I've already implemented both test cases locally and would be happy to submit a PR. I just want to clarify which approach the maintainers prefer: removing the instruction or adding the tests?

Thanks in advance,
Marcel

[github-actions]

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