I needed an aggregate function similar to array_agg() but which aggregates only unique values. As it turned out there is no convenient way of doing this. What I ended up doing instead was aggregating to JSONB keys and then converting a JSONB object to an array:
SELECT array(select jsonb_object_keys(jsonb_object_agg(mycolumn, true))) FROM ...
This works but doesn't seem to be the greatest user experience. I would like to submit a patch that adds array_unique_agg() function unless anyone has strong objections to this feature.