Fastest way to generate a random-like unique string with random length in Python 3

I know how to create random string, like:

''.join(secrets.choice(string.ascii_uppercase + string.digits) for _ in range(N))

However, there should be no duplicates so what I am currently just checking if the key already exists in a list, like shown in the following code:

import secrets
import string
import numpy as np


amount_of_keys = 40000

keys = []

for i in range(0,amount_of_keys):
    N = np.random.randint(12,20)
    n_key = ''.join(secrets.choice(string.ascii_uppercase + string.digits) for _ in range(N))
    if not n_key in keys:
        keys.append(n_key)

Which is okay for a small amount of keys like 40000, however the problem does not scale well the more keys there are. So I am wondering if there is a faster way to get to the result for even more keys, like 999999