Collect the matching replacements and distinguish all three possible multiplicities:
applyRule :: [Rule] -> Char -> String
applyRule rules c =
  case [replacement | (key, replacement) <- rules, key == c] of
    []          -> error ("no rule for " ++ [c])
    [result]    -> result
    (_:_:_)     -> error ("multiple rules for " ++ [c])
The list comprehension selects precisely the rules whose left-hand character is c. A singleton is the required unique match. The third branch also rejects duplicate rules with identical right-hand sides: there are still two matching rules. Finiteness of the rule list makes the uniqueness test decidable. The returned replacement may itself be empty.

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