There are several synonyms for the word "incumbently" that can be used interchangeably depending on the context of the sentence. Some of these synonyms include "necessarily," "inevitably," "obligatorily," and "essentially." Each of these words implies a sense of duty or requirement, emphasizing that something must be done or fulfilled in order to adhere to a certain standard or expectation. Additionally, words such as "compulsorily" or "mandatorily" could be used to convey a stronger sense of obligation or requirement, while words like "indispensably" or "vitally" could emphasize the importance or significance of fulfilling that duty or requirement.