How to generate a serial of unqiue password random numbers+letters in python?

I need to generate 10M (10000000) unquie random password everyday, per my code below,I will get 2~3 duplicated password in 10M.

1. can use set() to remove duplicated password.
2. tried to use a fixed letter (eg,A) as the first letter, then generators 7 random letters+ numbers, then use a fixed letter (B) as the second letter,etc.

length:12 prefer: 8 letter front,4 numbers.

Do you have any other solutions for this generating a serial of unqiue password?

from os import urandom
numbers = "23456789"
letters = "ABCDEFGHJKLMNPRSTUVWXYZ"
def generatePassword():
    password = "".join(letters[ord(c) % len(letters)] for c in urandom(8))
    password += "".join(numbers[ord(c) % len(numbers)] for c in urandom(4))
    return password
password_lists = []
for index in xrange(100000000):
    password_lists.append(generatePassword())

unqiue_password = set(password_lists)# removed duplicated password