A fresh install of the game would fix this, but that is if you actually uninstall FH completely as well.
99% of the time, if this ogre error tells you something is wrong with a certain file, it is.
So if it tells you there's already a particle called "RunicRing2" Then you must delete one of them inside the game directory. However, this one mentions it's a particle? Go into the my_objects or particles folder or whatevs and look for 2 similar files there.
If you don't find anything odd, just make backups off all your map folders or preset sturfff you have in the making and reinstall the whole thing(also uninstall otherwise it might not help at all). But if you do this, keep in mind that if you will just paste all custom objects and particles straight into your game folder again after a reinstall, chances you get the same error again will be very high.
But yarr, you shouldn't need to reinstall at all, just delete a particle called RunicRing2 inside the game folder. Though if you did just paste in objects and particles without caring what was there before, more errors like these might come?