23. If an individual does not know his or her eTA number (for example, if he or she wasn't told what his or her eTA number when receiving a work permit at a Port of Entry), would he or she then able to apply again for another eTA? Would the system prevent

Individuals who find themselves in this situation, or who simply wish to re-apply for a new eTA, may do so at any time. Once the more recent eTA is approved, it will replace the one that already exists. If the individual applies for another eTA, but on a different passport, that eTA - once approved - would not replace the one that already exists; it would be tied to the second passport, which the individual could use when travelling to Canada by air. In either case, the system will not prevent the individual from proceeding.