use strict;
use warnings;

use Data::Dumper;

my @input = qw{
   1200 1300 1200 1000 1100 1200
   1500 1700 2000 2100 3000 2100
   1200 1500 1700 1700
   };

my @groups = deDup( \ @input );

print Data::Dumper->Dumpxs( [ \ @groups ], [ qw{ *groups } ] );

sub deDup
{
    my $raToCheck = shift;

    my $raUniq = [];
    my $raDups = [];
    my %seen;

    push @{ $seen{ $_ } ++ ? $raDups : $raUniq }, $_ for @$raToCheck;

    return $raUniq, @$raDups ? deDup( $raDups ) : ();
}