-o     Overwrite previously existing files
  -o-    Never overwrite files

  This option will cause PKUNZIP to overwrite any files that already exist
  in the target directory, without prompting.  When this option is not
  specified PKUNZIP will pause if it attempts to extract a file that already
  exists.  It will prompt you:

     PKUNZIP: Warning! File XXXXXX already exists.  Overwrite (y/n/a/r)?

  If you forget to specify the -o when you give the PKUNZIP command, you may
  respond to the above question with "a" (All).  PKUNZIP will continue as
  though a -o had been specified originally.

  The -o- option will cause PKUNZIP to skip any file which already exists,
  regardless of file sizes or dates.