There is no way where we can control the dropdown dynamically as in the requirement. But, as a workaround we can do the following options:
Option 1) Color code the tab into Red color when the same employee is selected twice.
Option 2) There will be a "Submit" button after each assignment
Say, Position 1 -assign- Employee A --> Click Submit button (Emp A will not be seen for next)
Position 2 -assign- Employee B --> Click Submit button(Emp B will not be seen for next)
If we go with option 2, we should create a false employee list with mappings to actual employee list. And, use the false list for assignment. Whenever the "Submit" button is clicked, a process will update the false employee into position list and find actual employee using finditem. And, delete the particular employee from the false employee list in the same process.
If you are okay with option 2, i can help you further in buiding it.