This may show my ignorance but mrkl hash --> d5d27987d2a3dfc724e359870c6644b40e497bdc0589a033220fe15429d88599
Is found in block 91842 and 91812
I guess it's not clear to me exactly how the mrkl tree works, but I was under the impression that provided there wasn't a collision that these values would not be repeated.
When you look up block 91812 on block explorer you find this:
This transaction is an exact copy of http://blockexplorer.com/tx/d5d27987d2a3dfc724e359870c6644b40e497bdc0589a033220fe15429d88599
This is usually caused by flawed custom miner code that rarely changes the keys used by generations, and is therefore likely to produce a generation transaction with the exact same data as a previous one by the same person. The network sees duplicate transactions as the same: only one can be redeemed.