You are actually very very lucky! I just had my first double booking and I didn't catch it until a few hours BEFORE CHECKIN!!! It was pretty much a nightmare!
I would personally take the 15 night booking and cancel the other. You will need to contact the people from the 6th, apologize and refund ALL their money. At this point, I would think their damages are are slim since you are almost 9 months out. I would not offer them anything since they have ample time to secure something else. A kind gesture might be to search some other available properties with their criteria and send them the links. They cannot leave you a review because they never stayed at your property.
This advice would be totally different if you were closer to the check in date because at that point, it might be more difficult to find other accomodations.
I have mixed feelings about the recommendations outlned above.
I think it is fair to honor the reservation that was made first, as Sodamo suggests, but I would leave it up to you to decide which reservation to honor. The individuals involved will not know, but you will. You have to be comfortable with your decision.
I would provide comparable listings, as Sophie suggests, to the guest that you decide to cancel, but I would caution you make it clear you are not recommending the properties, unless you are familiar with the owner and the property. The list would be a starting point for the guest to begin the booking process, not a list of quick picks to find a property to substitute for yours.
An educational question?
How can anyone NOT honor the first booking? Seriously.
Honor the first and there is no reason for them to even know about the double booking. Hard as it is, you own up to #2 and work through it.
If one decides to take the 2nd booking because it is longer, what do you tell the 1st booker - "sorry, I got a better offer." Honest in a sense, but not good business IMHO.
I'd cancel the shorter term rental. What's the BFD? It's nine months out. You're in business to make money. Both groups were smart enough to find your VR using a listing site(s). They can find another place to rent by going back on the site. I'd check to be sure there are comparable rentals in your area and mention that but not suggest any particular rental. Just apologize to the shorter term renter, telling them you accidentally double booked. No need to say anything else.Yes, honestly is the best policy but everybody lies Unless there is nothing at all comparable, the only person hurt by this will be you earning less money.One caveat: the longer you wait to notify whichever one you cancel, the less time they have to have another VR so don't delay.