Thanks for the response. I agree that your threshold is not unreasonable, but it would be nice if it was stated in the problem. It does feel kinda of bad to have to run the code a few times until you pass based on how your Random Generator calls go. Or perhaps you could edit the error message from the test to says "There are > 10% duplicates" or something. Ideally, I'd love for you to add the 10% threshold in the challenge text and the error message.

In Java, getting a failed test saying "there are duplicates", followed by one of the randomly generated strings. The question does not say that they all need to be unique, only that they be valid. Add that to description or change this test.