It's not unusual for that to happen, which is why it's usually suggested to mark the eggs you set and remove any unmarked eggs on a daily basis.You could candle them and remove any with little or no development. How many eggs does she have? If it is her first time broody, I would reduce the number of eggs down to the most developed looking five or six and mark them now, so that you can identify and remove any new eggs from here on in.
The other option, if you have an incubator, would be to leave all the eggs under her and 2days after the first chicks have hatched, remove any unhatched and place them in the incubator to finish them off and sneak those chicks back under the broody at night once they are dry although if there are too many of them she will be unable to cover them all, so probably best to weed out the least developed ones now.
To candle them you just need a good torch and a dark room or thick towel over your head to block the daylight out. If you do some research on what to look for you should see a significant difference between the new ones that have just been recently laid and the 2 week old ones.
Good luck
Barbara