Maybe you missed one the day before or someone is playing a joke on you, but a second egg in the same day is possible. The egg starts its way through the hen's internal egg making factory when a yolk is released. Usually this is about once a day, but sometimes an extra one can be released early. If two are released at the same time, you could wind up with a double yolked egg. If there is some time between the releases, you can get two in a day.
Often, the second egg is soft or thin shelled. The shell gland only makes so much shell material in a day. If the second egg comes through early, that shell gland may not have had enough time to make enough material for a second egg. It's not always that noticable, but maybe your late egg was thinner-shelled than normal?
When they do this, they often skip laying the next day. Not always, but often.
If it is pretty rare, it is not a big deal. The hen's internal egg making factory simply messed up and released a second yolk too early. Stuff happens. If it happens on a regular basis, it could mean the hen's internal system is not quite right. This seems to happen more often with hen's fed a higher than normal protein diet.
