You only ever need to apply one role to any member of staff, and this should be their most senior role. Avoid duplicating a staff member in order to apply an additional role, as this could fragment the data they produce.
Diggit understands role hierarchies as follows:
Digger < Supervisor < Project Manager
So, each role can also operate as all that have come before it - meaning that a Project Manager can also behave as a Digger and a Supervisor.
If you have duplicated a staff member, it is recommended that you delete the version that has recorded the least data, and then edit the remaining version to assign the highest role they occupy to them. Note that deleting a staff member doesn't delete the data they have produced for you; it simply removes their name and initials from this recorded data.