Avoid double-initialisation of users

[kronosapiens]

The introduction of bf04fcf has added a race condition by which users are being initialised repeatedly. c566096 is a somewhat hacky fix. The issue should be fixed properly if possible, potentially through the introduction of a database constraint.

[kronosapiens]

A partial unique index may do the trick: https://knexjs.org/guide/schema-builder.html#unique

knex.schema.alterTable('job', function (t) {
  t.unique(['account_id', 'program_id'], {
    indexName: 'job_composite_index',
    predicate: knex.whereNotNull('account_id'),
  });