Let's suppose that I can create SHA256d collisions, and I use this ability to create two transactions that have the same hash, A and B. These transactions are not the same, but they hash to the same value.
I submit transaction A to the network, and wait until it has a few confirmations. Then, I submit transaction B. Will it get included into the chain? Will it overwrite transaction A?
If the answer to the previous two questions is yes, then what will happen if the block containing transaction B is reorganized out of the chain? Will transaction A be un-overwritten?