ARRAYUNIQUE() only works on arrays, such as in rollup fields. It does not work on lists that are really text strings. Your formula field is probably producing a text string. How are you generating the list initially? If there is a rollup earlier in the process, you can try putting ARRAYUNIQUE() there.
If the values come from a long text field, you can use a script to split the text string into a list and make the list unique.