coll.union

flex.coll.union(list1, list2)

WITH [1, 2, 3] AS a, [3, 4, 5] AS b
RETURN flex.coll.union(a, b) AS result

result
--------------
[1, 2, 3, 4, 5]

MATCH (d1:Document {id: 'doc1'})
MATCH (d2:Document {id: 'doc2'})
RETURN flex.coll.union(d1.tags, d2.tags) AS allTags

MATCH (u:User)
WITH collect(u.interests) AS interestLists
RETURN reduce(result = [], list IN interestLists | 
    flex.coll.union(result, list)
) AS allUniqueInterests

MATCH (p:Product {id: 123})
MATCH (similar:Product)-[:SIMILAR_TO]-(p)
RETURN flex.coll.union(p.categories, similar.categories) AS combinedCategories