I have not heard of any fakes of John Darling PTD. Nowadays most 2014 and later PTDs just don't hold the same kind of value as they used to. I think when they started releasing four at one time and changing the rules so frequently, the PTD train lost a lot of steam, and as a result, PTDs seem to be on their way out as the hottest type of pin. (There are some exceptions of course, but generally most don't get much higher than 35 dollars, and a lot are in the $20-25 range.) The John Darling PTD wasn't very popular when he was released, and stuck around for awhile. Plus he's an LE500, which don't hold nearly as much value over the long term as an LE300 or 400.
In my opinion, $20 for John Darling is a pretty good bargain, but not worryingly so.
As far as faked PTDs, the only ones I know of that are confirmed to have been faked are Zero, Kermit, and a few of the Tinks.