# here is an example to get you started in untested code.

my ($in, $out, %hash);
open $in, "<", $file_name; # you set the name somewhere else
open $out, ">", $out_file;
local $/ = "@:";           # set the input record seperator
while (<$fh>) {
    if ($hash{$_}++) {     # will be undef first time then >0
        print "Warning duplicate found\n$_";
    } else {
        print $out $_
    }
}