Single login form for multiple user

I'm working on a project which has many user type such as Contributor, Operator, CS, Admin with different roles and tasks. For now I separate login form just 2, one for Admin only and one another for the rest type of user. The problem for the time being I could not have same credentials for different type of user, for example if I have username "employee1" and password "emp1secreet" for type of user Contributor then I can't have same those username and password for Operator because single login form handles with single business model, I can't guarantee if person who log in is Operator or Contributor, but anything else it's doesn't matter. My question is "is common to have single login form for multiple user?" or I need separate them into each different login form/page? thanks for your help