From @MCEventHorizon on May 9, 2016 22:57

It doesn't happen without RTG so I think it has something to do with this mod.
Crashlog: http://pastebin.com/fCJCgndH

Copied from original issue: Team-RTG/1.9-Realistic-Terrain-Generation#102

From @topisani on May 11, 2016 9:28

This is the known biome dictionary bug. No idea whats causing it, but i will investigate.

The duplicate reffers to #78

this one was killed with the whole RBB != BGB thing. Thats ages ago.