Is an employer obliged to impose the same disciplinary action where two employees break the same rule?

No, there is no obligation on an employer to impose the same disciplinary action where two employees break the same rule. Each case should be decided on its own merits, and any sanction applied should take into account the individual employee's circumstances. For example, the employer should consider the employee's disciplinary record, his or her level of seniority and experience, and any mitigating circumstances. However, the employer should also consider whether or not it is acting consistently, and should impose different sanctions only where it is reasonable to do so.