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A few things make those 31.42BTC transactions unique:
+ The timestamps in them will be different.
+ The input transactions will be different (you can think of those as being different 'coins' going in to make the payment).
+ And if the input transactions don't add up to exactly 31.42 (and they probably won't), they'll have different output transactions for returning any change to Alice.
By the way: all that stuff is hashed together to give each transaction a unique 256-bit transaction ID (which you never see, but is used internally so Bitcoin can quickly figure out if it has already seen this transaction before).