Sorting with GNU Awk

I have an application were I need sorting. My tests led me to
this variant that works as I expected (with GNU Awk 5.3.0)

  asort (arr, res, "@val_num_desc")

But I originally intended an _inplace_ sort and I thought that
using these commands would also work

  PROCINFO["sorted_in"] = "@val_num_desc"
  asort (arr)

but the sorting order is (contrary to my specifier) ascending.
So it seems PROCINFO is not considered by the asort() function?
(Or is that just inappropriately used?)

Another question; is there a simple way for a _unique sorting_
like Unix'es 'sort -u'? - By simple I mean some setting, not a
programmed function like using associative array indexes, etc.

Janis