Usage of 非不 in Middle (?) Chinese
色生，生故計是我，非不生。


This statement comes for a Buddhist Sutra, Saṃyukta Āgama (雜阿含經). It is not easy to understand if one has no training in Buddhism.

As these words contains lots of logic in Buddhism translated from Pāḷi language, it is pretty obscure to most people. But it is required to distinguish everything in the logic of Buddhism.

You can view 非不生 this way. These is 不生 and one negates it to 非不生. It is like English, "impossible" and "not impossible". 

Those double negatives are quite common in Buddhist writing. If you know 心經, there is a phrase 無無明，亦無無明盡，乃至無老死，亦無老死盡。無無明 is the negation of 無明.