I've grown plenty of Hermaphrodites and I have a complete Thread dedicated to this subject. When I first dealt with some Herm's in my grow closet, I panicked! Fuck, my grow is FUCKED! Well, I over reacted! I get a Herm in my grow now, and I just pluck the balls off of the nodes (large grows can't do this, so I understand their frustration with Herms).
If you remove the pollen sacks/nanners when you see them, you'll have very few, if any seeds in the end. And the seeds you do get, they will be Female, with the Hermaphrodite tendency/trait. I have also grown plants, using the seeds from my Herm grow, with no nanners or balls. So it's not a guarantee that your plant will Herm even though you used the seeds from your Herm'd plant.
Breeders will force MJ plants to Herm so they can acquire the pollen that will guarantee them Female seeds. This is why some growers don't use Feminized seeds and start their grow the old fashioned way; grow out regular seeds and separate the Male and Female plants, then Clone. As long as a stressful event doesn't take place, the plants should not Herm.
I have good friends who do large, outdoor grows, and they stay away from Feminized seeds. Also, if you ever find seeds in your bag weed, it's from a Hermaphrodite, unless someone just ignored that ugly Male growing in their garden.