Fun (Bad) error message for duplicate definitions in the same toplevel

[Izaakwltn]

For example:

(coalton-toplevel

  (define-type BigType)

  (define-struct BigType
    (wow String)))

Compiling generates this error:

Execution of a form compiled with errors.
Form:
  (COALTON-TOPLEVEL
  (DEFINE-TYPE BIGTYPE)
  (DEFINE-STRUCT BIGTYPE
    (WOW STRING)))
Compile-time error:
  (during macroexpansion of (COALTON-TOPLEVEL
  (DEFINE-TYPE BIGTYPE)
  ...))
The value
  #S(COALTON-IMPL/SOURCE:LOCATION
     :SOURCE #<COALTON-IMPL/SOURCE::SOURCE-FILE {7026C97BB3}>
     :SPAN (20 . 41))

is not of type
  LIST
   [Condition of type SB-INT:COMPILED-PROGRAM-ERROR]