Answer: It is not essential that a person re-do the shahadah since he/she is already a Muslim. However, seeing that some time has past since the shahadah was taken and the person was not practicing Islam, it is highly possible that statements and actions would have emanated from him/her which were not in accordance to Islam. As such, to avoid all types of doubt, it is better that the person re-do the shahadah.
And Allah knows best.
Mufti Waseem Khan