On a production hen, with 24/7 light, their cycle gets messed up. Thus, they sometimes lay more than one egg in a 24 hour period. Such as, one in the morning and one at night. If this is the case, remove artificial lighting and replace with natural. Not only will it straighten out your hen, but it'll also be healthier for both hens. The 24/7 lighting can be dangerous for the hens' overall health.
Is one of the eggs soft-shelled? In rare cases (I had it happen with my Blue Andalusian mix) a hen will lay two eggs at one time. One hard-shelled, one soft-shelled. In this case, it's a double yoker, but in two eggs instead of one. Again, this is very rare. Though if happening, try giving your hen oyster shells and keep track of your hen for any health issues relating to egg production.
On another note.... Are you sure that there's no other hens getting in? Such as, your hens are free ranging and a neighbor's hen is laying in your nesting box. Perhaps, you should quarantine both hens to find the extra egg layer. Maybe you could put a camera in your coop to see if another hen is laying in there, or if someone is actually playing a trick on you.