It is possible. If she releases two yolks at the same time, you can get a double-yolked egg. If there is less than 24 hours time difference in releasing the yolks, you can get two eggs the same day. It's not as uncommon as some people may think.
When this happens, one egg is often thin-shelled or soft-shelled. The shell gland can only make a certain amount of shell material in a time period and the egg has to spend a lot of time in the shell gland getting the shell material deposited. Sometimes there is not enough difference inthe shell thickness to be obvious, but this is often the cause when people get a soft-shelled egg when they don't expect it. It has nothing to do with how much calcium the hen is getting. It's just that she started a second egg too soon.
Often, they will skip laying an egg the next day when they do this. Not always, but often. It sounds like yours is not laying every day anyway so you probably would not notice.
Another possible reason for this is something as mundane as you did not find the egg the previous day. I've had hens lay just before going on the roost, so it may have something to do with when you look for the eggs.