empty default duplicates handler

[Kevin Ryde]

Is it supposed to work to set the default duplicates handling to an
empty list?  Eg. foo.scm containing

        (define-module (foo)
          #:duplicates ()
          #:use-module (srfi srfi-1))
        (pk map)

It seems to work in 1.8, but in the head it's giving

        => Unbound variable: map

I had this in my program as a global setting

        (default-duplicate-binding-handler '())

Either way if empty isn't allowed it'd be nice to throw an error
immediately, the same as a bogus handler name symbol does.

The effect I wanted was no work at all done for duplicates checking.
Modules can override/extend the core by shadowing, but I'm confident
there's no clashes between my modules and don't want time spent looking
at that.  Or is '(last) the policy I should be asking for to get that
non-checking effect?