You can build a list of employees with no duplicates using the following parameters:
Objects:
User Base
...User Role
...User Role [1]

Global Filters: (make sure to include the parentheses)
Where ( User Role.Role is any of Teacher
Or User Role [1].Role is any of Non-Teaching Staff )

Object Filters:
And User Role.Role is any of Teacher
And User Role [1].Role is any of Non-Teaching Staff