I did a search for RR knives and Double R knives online. I found several custom knife makers with the initials RR, but didn't see any pictures on their websites with this mark. Some makers sometimes use different marks for one reason or another though. Ben Tendick on here is a good example. It could be a different mark from one of the ones I found, but no way to know for sure. Plus there are a lot of makers that don't have websites.
Matt