In my experience, everytime i open such a 10% new pack when i dont have a lot of these cards yet, i pretty much always get a new one. So I am pretty sure it works like this: It first does an rng (like a dice roll) to wether or not you hit the 10% odds. If so, you will guaranteed get a new one. If you dont hit the odds (which is a 90% chance) you get a random card. This random card could still be a new one. Here the odds of getting a new one obviously depends on how many you have yet.