Fix duplicate orgs in the piggyback orgs list

If clojure.set/union is passed a general seq and a set it doesn't
actually perform a set union, instead it returns a list of all the
entries in its argument collections.
Merge pull request #3641 from circleci/fix-duplicate-piggyback-orgs

Fix duplicate orgs in the piggyback orgs list
Merge commit 'dac3235e19d49daf19b21a103aa62aec50b8c05f'