The chance of rolling any given double (e.g., as in your link, a double 6) is indeed 1∕36. There are, however, six doubles to choose from, so the total probability is in fact 1∕6 of rolling any double.
The chance of rolling a single is 1/6 ⁓ but two dice thence double the likelihood of instances to 1/3
Alas, adding probabilities does not work that way. In effect you've counted the outcome of a double twice. The chance of rolling at least one of a given number with 2 dice is, in fact, 1−(1−1∕6)²=11∕36.
The chance of rolling any given double (e.g., as in your link, a double 6) is indeed 1∕36. There are, however, six doubles to choose from, so the total probability is in fact 1∕6 of rolling any double.
Alas, adding probabilities does not work that way. In effect you've counted the outcome of a double twice. The chance of rolling at least one of a given number with 2 dice is, in fact, 1−(1−1∕6)²=11∕36.