In the case of a serial complainant, is the company obliged to hear multiple grievances which are believed to be false when there are concerns about the mental stability of the complainant?

There are no strict legal rules which apply when an employee raises multiple grievances. If the grievances are related, an employer might be in a position to refuse to hear a new grievance on the grounds that the matter has already been the subject of a grievance procedure. If the grievances are unrelated, an employee is expected to act reasonably and to take all steps that are required as a reasonable employer to investigate the grievance. If the employer has a concern regarding the mental stability of the employee, the employer could consider whether it is appropriate to ask the employee to undergo a medical or occupational health assessment before engaging in the grievance process.