Something went wrong in the Beast lightmapper: Duplicate name

The full error:

Something went wrong in the Beast lightmapper: Duplicate name - An instance named New Terrain-TreeInstance-0 already exists
 This is a bug - please report it with a scene reproducing the issue.

UnityEditor.DockArea:OnGUI()

It uses “tree_small_e” and “tree_small_d” from the BootCamp complete project.

Just found out, seconds after I posted this question, that I had somehow managed to duplicate the terrain.