verifier.py: fix CVE-2012-2459: reject left-sibling duplicates

[SomberNight]

Due to how the txid-commitment merkle tree used in the block headers is constructed, we need an extra check to be able to validate the position of a txid in a block.

I think this is low severity for us.

See https://bitcointalk.org/?topic=102395 :

The Merkle hash implementation that Bitcoin uses to calculate the Merkle
root in a block header is flawed in that one can easily construct multiple
lists of hashes that map to the same Merkle root.
For example, merkle_hash([a, b, c]) and merkle_hash([a, b, c, c]) yield
the same result. This is because, at every iteration, the Merkle hash
function pads its intermediate list of hashes with the last hash if the
list is of odd length, in order to make it of even length.

And so, the Merkle root function can be effectively preimaged by
changing the input so that one of the intermediate lists is of even
length with the last two elements equal (where originally it was
of odd length with a last element equal to the earlier mentioned two).
As was later noted, this extends to any input length that is
not a power of two:
merkle_hash([a, b, c, d, e, f]) == merkle_hash([a, b, c, d, e, f, e, f]).
Note that to maintain the same root hash, the only flexibility that
exists is duplication of elements.

Ported from Electron-Cash@1651463