`(parent_left, parent_right)` duplicates can occur when creating multiple
manufacturing orders in parallel, which mess up the result of the `child_of`
search operator (wrong result returned).
Related Odoo issue: https://github.com/odoo/odoo/pull/14515
Odoo fix (for `stock.quant.package` here): https://github.com/odoo/odoo/pull/14694
This issue should not be relevant anymore in 11.0 as the `child_of/parent_of`
mechanism has been refactored (https://github.com/odoo/odoo/pull/22558)