use strict;
use warnings;

my %folded_lines;
while ( <> ) {
    chomp;
    push @{$folded_lines{$_}}, $.;
}

my @dupes = sort { $folded_lines{$a} <=> $folded_lines{$b} }
            grep {    @{ $folded_lines{$_} } > 1    } 
            keys %folded_lines;

print RESULT_FILE $_, ': ', join( ', ', @$folded_lines{$_} ), "\n" for @dupes;