Grevit update not working

Hi

I'm trying to export walls from Grasshopper to Revit using Grevit. The walls are a simple polygon in plan and controlled by a nominal height. I can export the walls fine to Revit but if I try to update the geometry within Grasshopper and re-export, I get duplicate walls. The update Boolean is set to 'True' so I'm not sure why this is happening. The walls are simple lines, no NURBS. I've tried with polyline curves and individual curves but am still getting the same error. What would be causing this? 

Thanks