In two ways. First, there are bound to be more than one Bitcoin address bound to certain keys. Easy to fix this one by increasing the amount of number slots. Second, by using a smaller pool of characters, you are more likely to generate the same address multiple times. This is why Bitcoin uses base58. If characters that look similar to you are not a problem, you could expand it to base64, increasing the address possibilities significantly. Taking it a step further, which would add 0, O, I, l you could add other characters like !@#$%)^()& to add even more