The term "hermie" is misused and misunderstood.
A real hermaphrodite is rare, except in some landraces. A real hermaphrodite has to have an X and a Y chromosome. That means it is dioecious. A real hermaphrodite will exhibit male and female flowers together. A monoecious hermaphrodite will exhibit the flowers together too, if the plant is stressed from the get go. For the most part, a monoecious hermaphrodite will have separate male and female flowers.
What 99.99% of people are talking about when they say "hermie" or hermaphrodite is a female plant that has produced male flowers. Technically this is not a hermaphrodite. When speaking of breeding with feminized seeds someone referred to a female plant that has been forced to produce male flowers as a "Trannie." That is a much more apropos name.
When you have a female plant, it will have two X chromosomes. It can not become a hermaphrodite. A real, dioecious hermaphrodite has to have an X and a Y chromosome. If a real female plant (XX) produces male flowers it is still a female. The male flowers are a normal and natural survival mechanism. If cannabis did not have this, it probably would not exist. While many curse "nanners" I think they are a blessing and a great aspect of a wonderful plant.
Male flowers on a female plant can be induced in different ways. Stress is the most common factor. Improper feeding, heat and over watering are the most common ways people stress plants.
You can also use chemicals to safely force a female plant to produce male flowers. These chemicals do not stress the plant, they just trick the plant into releasing the hormones that make male flowers. With colloidal silver, you can spray one branch and only that branch will produce male flowers.
The pollen from either the balls or the nanners of a female plant will only have X chromosomes. So when that pollen makes a seed it will be a feminized seed. A feminized technically can not produce a hermaphrodite, since it does not have a Y chromosome. A feminized seed can produce female plant with male flowers, just like a female plant from a regular seed.
Plants from properly made feminized seeds are 99.999999% female and are just as likely to produce male flowers, hermie, as plants from regular seeds.