You ask. Where is it repeated? The answer is that four fives can't be arranged, and no more elements containing five can be arranged. Four fives are all the same, only one can be counted.

So, there are only two options. One is to arrange the positions for the four 5s first, and there are C (4,6) ways to choose the positions. 5s has only one filling method, and the remaining two positions are selected from eight at a time.

Second, first choose two of the six positions to arrange numbers other than 5, and then arrange the remaining four positions as 5. Note that there is only one scheme in permutation 5.