Vague `grade-school` exercise instruction requirement

[akilegaspi]

The last paragraph reads as:

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

What does roster mean here? a particular grade's roster or the entire school's roster.

If the edge case is encountered, what is the expected emitted error? Would it be through the type system as a result or an exception?

[github-actions]

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