How to get a different hexadecimal value from the previous value generations?

import os,binascii
print binascii.b2a_hex(os.urandom(1))

I am creating a hexadecimal value using os.urandom as you can see above, I need to generate 15 different values.

I got as far as generating the values, but I don't know how to make it so that all the values are different from the values generated before. Any help is appreciated.