Clarify unknown keys warning
Remove uniqueness constraint on :id

Multiple options for the same :id makes some sense:

  [["-v" "--verbosity"
    :default 3
    :parse-fn #(Integer/parseInt %)]
   ["-q" "--quiet"
    :id :verbosity
    :parse-fn (constantly 0)]]

However, multiple :default entries for the same :id does not make sense,
so it is prohibited.

The identity constraint on :id remains.