Steps to reproduce:
1. Open the attached project and load scene TestScene01
2. Select Terrain from the hierarchy
3. Begin modifying the terrain using the sculpt tools
4. Notice that the second terrain gets modified as well due to it sharing the same TerrainData asset

Expected behavior: When terrain is duplicated (or a scene containing a terrain is duplicated), a new TerrainData asset should be created.

Actual behavior: Duplicated terrains end up sharing the same TerrainData asset which creates problems when modifying terrains. In addition, it causes issues with multi-scene editing and baking.