added comment to avoid this vulnarability sneaking back

core/src/core/mod.rs

@@ -164,6 +164,8 @@ impl Hashed for HPair {
 }
 /// An iterator over hashes in a vector that pairs them to build a row in a
 /// Merkle tree. If the vector has an odd number of hashes, it appends a zero hash
+/// See https://bitcointalk.org/index.php?topic=102395.0 CVE-2012-2459 (block merkle calculation exploit)
+/// for the argument against duplication of last hash
 struct HPairIter(Vec<Hash>);
 impl Iterator for HPairIter {
 	type Item = HPair;