(a) A teacher who (i) has been receiving a disability, occupational disability, or disability retirement benefit under Section 16-149, 16-149.1, or 16-149.2 for at least one year and (ii) remains unable to resume regular full-time teaching due to disability, but is able to engage in limited or part-time employment as a teacher, may engage in such limited or part-time employment as a teacher for an employer under either this Article or an employer under Article 15 of this Code without loss of the disability, occupational disability, or disability retirement benefit, provided that the teacher’s earnings for that limited or part-time employment, when added to the amount of the benefit, do not exceed 100% of the salary rate upon which the benefit is based.
     (b) A disabled teacher who engages in limited or part-time teaching under this Section and earns service and contribution credits for that teaching shall not receive duplicate service or contribution credits under Section 16-149 or 16-149.1.