Using 10 characters: In this case, 1 character can be used twice. To determine the number of possibilities, let’s first pick which character can be doubled. There are 9 options for picking that character. Next, if we consider the two instances of that letter as distinct values then we can just count the number of ways to arrange unique 10 characters which is 10! However, swapping the two characters (which are actually identical) would not give a new passphrase. Since these are counted twice, let’s divide these out to give 10!/2.