I was working on one of the maps for my game and finished it, I usually build maps on separate baseplates from my main game because I find it much easier to navigate through the explorer etc. When I imported the map onto my game the terrain (made from meshparts) had this lighting glitch.
Note: I have fixed the issue
For anyone who stumbles across this issue in the future it seems as though the double sided feature in the properties tab was effecting the shadows of the meshes causing them to duplicate so the solution was to go in to blender and flip the normals myself, I found this useful YouTube tutorial teaching how to do so
