R5RS says it's an error for a key to appear in more than one clause of case (or twice in the same clause, but that's trivial). R6RS allows the same key to appear more than one clause, but insists on left-to-right testing throughout, like cond. The R6RS editors thought this was better for machine-generated code, though worse for hand-written code.
I suggest a compromise: allow keys to appear in more than one clause, but behave as if the key appeared only in the first (leftmost) clause. This allows hash-table or other non-left-to-right implementations.