Because if this mode of transfer ever becomes more than a toy, then there will be massive overloading of those values.
Can you come up with circumstances in which someone might choose to make successive transactions of the same value? Especially if that might confuse the issue?
You want to be able to tell if _this_ transaction completed, not if, say "A transaction that really looks like this one" completed.
No. "successive transactions of the same value [between the same people]" will not have the same transaction inputs. A 'transaction input' in bitcoin isn't the originator's pubkey, it's the output of a previous transaction. A second transaction which literally had the same inputs as a first would be a doublespend attempt.
Two transactions with the same inputs and the same outputs can safely be considered the same transaction. You don't just compare the transaction value.
Can you come up with circumstances in which someone might choose to make successive transactions of the same value? Especially if that might confuse the issue?
You want to be able to tell if _this_ transaction completed, not if, say "A transaction that really looks like this one" completed.