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file 1: H(15)
        H(16)
file 2: H(15)
        H(15)
        H(16)

foreach my $key (@hydrogen_split) {
$hydrogen_split{$key}++;
}
...
foreach my $key ( sort grep { $hydrogen_split{$key} > 1 } keys %hydrog
+en_split){
print "$key\n";
}