use strict;
use warnings;

my %entries = ();
while (<DATA>) {
    chomp;
    if (/^\@:\d+/) { #if line starts with aht-colon-digit(s)
        if (exists $entries{$_}) { #$_ is the current line
            #$. is the current line number
            print "Line $. duplicates line $entries{$_} [$_]\n"; 
        }
        else
        {
            $entries{$_} = $.;
        }
    } 
}
1;
__DATA__
@:1107530184::1
kkkkkkkkkkkkmkmkmk
kkkkkk
confused.gif
@:1107530257:1107530439:1
kmkmkm
<br>kmkmkm
<br>
<br>Fri Feb  4 10:17:37 2005
<br>
mad.gif
@:1107530709::1
ygyg
ygygygyg
lol.gif
@:1107530717::1
ygyg
ygygygyg
lol.gif
@:1107530963::1
cool
help
cool.gif
@:1107532649:1107532689:1
k
<br>kkkkkkkkkkkkkkkkk
<br>
<br>Fri Feb  4 10:57:29 2005
<br>
lol
lol.gif
@:1107530257:1107530439:1
kmkmkm
<br>kmkmkm
<br>
<br>Fri Feb  4 10:17:37 2005
<br>
mad.gif
@:1107532758::1
lll
Lets
mad.gif
@:1107532976::1
lll
Lets
mad.gif