perl -e '
for $i (0..3) {
  $sub   = "xxx";
  $par   = "xxx " x $i;
  $count = () = $par =~ /\Q$sub\E/g;
  $msg   = qw( none one duplicates ) [ $count<2 ? $count : 2 ]; 
  print $msg,"\n";
}'
none
one
duplicates
duplicates