Well, Fibery doesn’t really have the concept of arrays, except as entities in a database.
So there is no such thing as the ‘unique values in a collection field’ since all entries in a collection field are, per definition, unique.
Of course, they might have identical values in one of more of their fields (which seems to be the case you’re interested in).
If the field by which uniqueness is determined is a select field, or a collection field, then the lookup method described above will work.
Otherwise, it’s not possible, since there is no valid datatype for storing the result.
So for example, if it is a number field, and you want all the possible unique values of that number field, the result would be a collection (array) of numbers (which is a primitive datatype) but there is no way to store an array of primitives.
I hope that makes sense.
It’s not meant to be an attempt to undermine/disagree with what is a clearly-defined need, just an explanation of why it is not currently possible.