I think this has to be a glitch of some sort.

You can do a one to one comparison of your father to your father-in-law to see if it's a 100% match. Same with your mother to your father-in-law. If one of them matches 100%, then it would appear that one of the files was uploaded twice (to one of your parents and to your father-in-law). Since the DNA data is from ancestry, it's simply labelled in a zip file on your computer with the date. Did you download two or all three on the same day? If so, then it would be very easy to mix up files when loading to gedmatch.