_id is absolutely unique on Kobo (within a particular server, e.g. _id will never be duplicated on kf.kobotoolbox.org). It is literally the primary key of a relational database table.
_uuid should be unique according to the ODK specifications, but sometimes is not. We’ve prioritized accepting data from clients even if it contains duplicate UUIDs. That is changing, and we will soon reject any submission that duplicates a UUID, even if the rest of the submission content differs. However, we’ll also have to deal with de-duplicating UUIDs among existing submissions—and this is not a trivial process.
Thanks.
