I know this thread is old, but I came to it first, and then found an example of a transaction hash being in multiple non-forked blocks in the block chain. The blocks that contain the transaction with hash

d5d27987d2a3dfc724e359870c6644b40e497bdc0589a033220fe15429d88599

are 91812 and 91842. As long as the previous transaction outputs have all been spent, this double transaction hash is considered valid. If any of the addresses that received BTC from the transaction in block number 91812 hadn't spent their coins yet, this would be invalid.