It depends on the encryption scheme used.  For a stream cipher (including AES 
in counter or OFB mode), this yields the keystream.  If someone screws up and 
uses the same key and IV twice, you can use knowledge of the first plaintext to 
learn the second.  For other AES chaining modes, it's less scary, though if 
someone reuses their key and IV, knowing plaintext xor ciphertext from the 
first time the key,iv pair was used can reveal some plaintext from the second 
time it was used.  

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