This script identifies the duplicate lines with their corresponding line numbers within a file.

Dupline

#!/usr/bin/perl

#	This program is free software: you can redistribute it and/or modify
#	it under the terms of the GNU General Public License as published by
#	the Free Software Foundation, either version 3 of the License, or
#	(at your option) any later version.
#
#	This program is distributed in the hope that it will be useful,
#	but WITHOUT ANY WARRANTY; without even the implied warranty of
#	MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
#	GNU General Public License for more details.
#
#	You should have received a copy of the GNU General Public License
#	along with this program.  If not, see <http://www.gnu.org/licenses/>.
#
#	Copyright (c) 2010 Ruel Pagayon <ruel@ruel.me> - http://ruel.me

use strict;
use warnings;

sub loadf($) {
    my @file = ( );
    open(FILE, $_[0] . "\n") or die("Couldn't Open " . $_[0] . "\n");
    @file = <FILE>;
    close(FILE);
    return @file;
}

{
	my @file = loadf("path-to-file.txt");
	my @inner = @file;
	my @dup = ( );
	my $l0 = 0; my $l1 = 0; my $l2 = 0; my $dc = 0;	my $tc;
	foreach my $line (@file) {
		$l1++;
		$line =~ s/^\s+//;
		$line =~ s/\s+$//;
		foreach my $iline (@inner) {
			$l2++;
			$iline =~ s/^\s+//;
			$iline =~ s/\s+$//;
			next if ($l1 == $l2 || grep { $_ eq $l1} @dup );
			if ($iline eq $line) {
				$dc++;
				if ($dc > 0) {
					if ($l0 == 0) {
						print "Line " . $l1 . ": " . $line . "\n";
						$l0++;
					}
					print "Line " . $l2 . ": " . $iline . "\n";
					push (@dup, $l2);
				}
			}
		}
		print "\n" unless($dc == 0);
		$dc = 0; $l0 = 0; $l2 = 0;
	}
}

__END__