ARRAYUNIQUE does not appear to work on text strings that have spaces. For some reason, it seems to work on text strings that are a single unbroken word.
I really need this for rollup fields that read in data from a junction table, as some of the data repeats. The rollup lists for a single record can be quite long, and the rollup fields are for humans to glance at and use to make decisions, so having several repeating strings makes it a lot easier for a person to overlook important data.