"This is not about me being afraid of losing business to this imposter, but more about the uniqueness of my property being destroyed by someone too lazy to come up with their own ideas jumping on the backs of those who have done all the work!"
@Bernie92, I spent a long period in Guatemala and got to know a couple indigenous vendors of handmade goods. Many main streets are lined with vendor stalls overflowing with textiles, it's just an explosion of color, and often the same items repeated everywhere. One of the vendors would complain about how every time she would design a unique new style of clothing with local fabric, it got lots of attention from tourists but only stood out for literally a couple of days, just long enough for the other vendors to copy the design and display theirs! The same thing would happen with the little convenience stores, as soon as one person opened one, another would open one right next door.
The competition was so fierce, but instead of each person having success with their own unique ideas and providing variety to tourists, and fairly competing with each other based on originality, the market would instantly get saturated with copies. No one ever got a leg up and experienced genuine success. However I don't think that will happen to you, thank goodness he's 25km away!
I can understand how it felt creepy. You welcomed a guest, and there he was going behind your back and taking measurements, photos and notes, all your hard work that was personal and came from your own creative vision. That's an almost intimate invasion.
You're a superhost with amazing reviews. While his place may look nearly identical as you say it does, your property is unique and your treatment of guests is special -- I love that one of the first reviews mentioned having scones by the stream. I didn't read the listing, but if you felt it would be of benefit, you could put at the beginning of the intro something like "First of its kind in the area, my original design," and maybe also a little blurb about how it came about and what the vision was. Of course, he may just change his listing to do the same, so perhaps ignoring him as if he doesn't exist is the best revenge. Ooh, or if you sketched the design or scrapbooked, you could include photos of that, or make a little scrapbook for yourself and a copy for the cottage, which guests could enjoy. Then whenever you get irritated with him, you can look back at all you did to make your vision come to life and bask in the glow of your personal awesomeness. It's not about him at all.
Finally, here are two quotes from Epictetus that might bring some comfort and/or perspective:
"This is why I lost my lamp: because a thief was better than I at staying awake. But he bought the lamp at a high price. In return he became a thief, he became untrustworthy...This seemed to him a good bargain!"
"Is a little oil spilled, a little wine stolen? Say 'this is the price of equanimity, this is the price of peace of mind' - for nothing comes free."
I hope you get some great responses here.