How many fertile eggs is about normal?

I have 2 silkie hens, atleast I think they are both hens, and a few weeks ago shes started brooding and rarely ever comes out of the coop. I was collecting her eggs for awhile hoping that she'd eventually stop but she never did so then we kinda stopped collecting them because she would rarely lay any. Well then a few days ago my other silkie started brooding also and so I went in today and collected 4 eggs. I thought I looked at them really good with a flashlight but obviously I didnt know what I was looking for because when I was throwing them in my sink, the first one had a baby in it that looked about 10 days old... I started crying and ran the other three eggs back to the chickens and they got right back on them..

My question is, is it possible for her to have layed more than one fertile egg??

The story of how it probably got fertalized: I have 8 other chickens, 2 of them roosters and they are all in a seperate coop from my 2 silkies. One of the roosters kept jumping on her while i was outside with them and I thought I had kept them away pretty darn good, we ended up getting rid of the one that was jumping on everyone about a week ago (he was very mean.) AHHH I feet TERRIBLE though! I hope she has another baby in the batchhh.

Maybe someone can also explain to me how to see if an egg is fertalized?? What do you use? Flashlight? Lightbulb???