Postgresql 9.3: Add Unique constraint using an existing unique index
You don't specify the column for the unique constraint. That's not necessary because the column list is defined through the index:

ALTER TABLE package ADD CONSTRAINT 
     unique_package_id UNIQUE USING INDEX package_tmp_id_idx;


There is an example for that in the manual:
http://www.postgresql.org/docs/current/static/sql-altertable.html

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