Nichiren's One Buddha Doctrine a recurrent theme

"In volume one of On “The Words and Phrases,” we read: “There are not two Buddhas in the world, there are not two rulers in the nation. In the realm of one Buddha, there are not two Venerable Ones.”

And The Treatise on the Upholding of the Stage states: “In one world there are not two Buddhas, in one nation there are not two rulers. Within the realm of a single Buddha’s teachings, there are not two Venerable Ones.”