Challenge: One table associated with several fields - but avoidin

I'm using the newest version of ACCESS. I have a (table) list of employees.
I have a form with 4 positions (clerk, door, clean-up, charge) that I want to
schedule using the same list of employees. How do I use this same list so
that if I choose an employee for clerk, that when I fill in the employee for
door - that employee can not be chosen again (i.e. I want to avoid the error
of scheduling the same employee for two or more positions).

Thanks! hope that made sense.