Issue:
When using the Material Handler option in the Inventory setup, users are seeing multiple duplicate Warehouses in the screen drop down lists.

Resolution:
This will happen if there are duplicate entries for the warehouse and employee in the IN_Material_Handler_Warehouses table.
--- Find the User_Id ---
select * from SY_Employee e
inner join SY_Users u on u.User_Id = e.User_Id
where u.User_Code ='BSP'
--- Find the Warehouse_Id ---
select * from IN_Warehouse where Warehouse_Code = 'ABC Service Co'
--- Find the Material Handler Records ---
select w.Warehouse_Code,Employee_Id,h.Warehouse_Id,*
from IN_Material_Handler_Warehouses h
inner join IN_Warehouse w on w.Warehouse_Id =h.Warehouse_Id
where h.Employee_Id = 59

You should remove all but one record for each warehouse.
--- Backup the Material Handler Records ---
Select * into IN_Material_Handler_Warehouses_Backup from IN_Material_Handler_Warehouses
--- Delete extra Material Handler Records ---
Delete from IN_Material_Handler_Warehouses
where Material_Handler_Warehouse_Id in(241,242,243,244,245)