The question is "if you flip a coin twice, and at least one coin lands heads, what is the probability both coins land heads?" This is a conditional probability question, meaning in calculating the probability both coins land heads, we consider only the cases where at least one coin landed heads. If you flip two coins, there are four equally likely outcomes: heads+heads, heads+tails, tails+heads, and tails+tails. We ignore the tails+tails outcome because of the condition. Out of the remaining three outcomes, there is only one in which both coins landed heads. Therefore the answer is 1/3.

Be warned that there are very similar-sounding questions to which the answer is 1/2. For example, consider the scenario in which two coins are flipped, and I randomly choose one of them to look at. Given that it is heads, what is the probability the other coin is heads? This seems like the same question as our original question, but it is not. Here we have to consider eight equally likely outcomes: heads+heads+picked_left, heads+heads+picked_right, heads+tails+picked_left, heads+tails+picked_right, tails+heads+picked_left, tails+heads+picked_right, tails+tails+picked_left, and tails+tails+picked_right. Our condition that the seen coin be heads narrows the possibilities down to four outcomes: heads+heads+picked_left, heads+heads+picked_right, heads+tails+picked_left, and tails+heads+picked_right. Of these four outcomes, there are two in which both coins are heads.