If you're trying to help your chosen people move out of a country by killing the firstborn of everyone else in the country, you might have a small conundrum:
You have promised your chosen people that you would kill all the firstborn except their own. You know that you will not have to kill a child in every family because of high mortality rates: there are not as many firstborn alive as would be expected. And you know that you will be sparing the firstborn of those other than your chosen people: some of your chosen people will convince their neighbors to put blood on the door frames, and you are willing to overlook this exploitation of a loophole.
However, there are certain people whom some argue that you should kill and others argue that you should not. These people had a father or mother who had children before, but are the first child of their other parent. Some of them are the result of men taking in multiple wives, some are the result of remarriages, and others are the result of adultery. They can be argued to be either firstborn or not, which ties in to whether you should kill them or not.
What should you do?