Let's say you roll 2 dice one at a time. There are 36 possible outcomes. You always had a 1 in 36 chance of rolling one of these combinations. That means that rolling a 6 then a 6 was always 1 in 36.

After you roll the first dice, you can no longer roll the other 5 numbers on the first dice, so as a result the chances of 30 of the combinations coming up is now 0%. The remaining 6 combinations are now 100%, up from 16.6% before the first roll.

The logical fallacy is that rolling a 6 and a 6 was always a 1 in 36 chance. But after you've rolled the first dice, you're not just asking if you're going to roll a 6 again. You're also asking what's the probability that I'm going to roll a double, which should be a lower probability right? The problem is that rolling 2 6's wasn't the only possible double before you rolled the first dice. You always had a 6 in 36 chance of rolling a double (1, 1 or 2, 2 etc). After rolling that first dice, you only have 1 chance to roll a 6 but there's no longer 36 possibilities, there's only 6.