Here is my attempt at this:
The word "HIGHMOON" has 8 letters. If you allow all letters to be permuted, in which case you will have "duplicates", you would get:
8! =40,320 permutations.
If you exclude duplicates, then you would have:
8! / 2!.2! =10,080 pemutations
However, in either case, you will have 2 "Hs" in your permutations, and since each letter appears an equal number of times in the total of ALL permutations, it therefore follows that in 2/8 =1/4 of ALL permutations, you will have 2 "Hs" next to each other. The other 6/8 = 3/4 of ALL permutations, they will NOT be next to each other. So, the answer is, I think, that in 3/4 or 75% of ALL permutations, they will NOT be together.
Note: somebody should check this. Thanks.