(and (or (eq? old new) (not replace2))
                 old)
            (and replace2 new))))

    (define (warn-override-core module name int1 val1 int2 val2 var val)
      (and (eq? int1 the-scm-module)
           (begin
             (format (current-warning-port)
                     "WARNING: ~A: imported module ~A overrides core binding `~A'\n"
                     (module-name module)
                     (module-name int2)
                     name)
             (module-variable int2 name))))

    (define (first module name int1 val1 int2 val2 var val)
      (or var (module-variable int1 name)))

    (define (last module name int1 val1 int2 val2 var val)
      (module-variable int2 name))

    (define (noop module name int1 val1 int2 val2 var val)
      #f)

    (set-module-name! m 'duplicate-handlers)
    (set-module-kind! m 'interface)
    (module-define! m 'check check)
    (module-define! m 'warn warn)
    (module-define! m 'replace replace)
    (module-define! m 'warn-override-core warn-override-core)
    (module-define! m 'first first)
    (module-define! m 'last last)
    (module-define! m 'merge-generics noop)
    (module-define! m 'merge-accessors noop)
    m))

(define (lookup-duplicates-handlers handler-names)
  (and handler-names
       (map (lambda (handler-name)
              (or (module-symbol-local-binding
                   duplicate-handlers handler-name #f)
                  (error "invalid duplicate handler name:"
                         handler-name)))
            (if (list? handler-names)
                handler-names
                (list handler-names)))))

(define default-duplicate-binding-procedures
  (case-lambda
    (()
     (or (module-duplicates-handlers (current-module))
         ;; Note: If you change this default, change it also in
         ;; `define-module*'.
         (lookup-duplicates-handlers
          '(replace warn-override-core warn last))))
    ((procs)
     (set-module-duplicates-handlers! (current-module) procs))))

(define default-duplicate-binding-handler
  (case-lambda
    (()
     (map procedure-name (default-duplicate-binding-procedures)))
    ((handlers)
     (default-duplicate-binding-procedures
       (lookup-duplicates-handlers handlers)))))