I'm curious as to what makes you say it isn't when it very well could be one. It is very obviously raised and rounded,not flattened. The doubling also resembles that on the South Dakota DDR or even the 2012 Denali quarter.
Granted the doubling on those coins is towards the middle of the coin, it's still obvious that it's not impossible to show up on other areas of a coin. I'd like to know if anyone has found something similar in their own collections, that they may have otherwise overlooked because they only focused on the doubled dies that had already been listed. I've visited all the sites listed above already. I've also made discoveries on coins, which I kept and stored before they were listed on those sites because I knew what I'd found then too.