I have a dataset of offices and employee sales within that office. An employee shouldn't appear in 2 offices. However, an employee id can appear multiple times within an office.

I want to check whether any employee_id appears in more than one office_id. (EG: whether employee_id 123 appears in BOTH office_id ABC and office_id DEF).

The following example code creates three ids: An office_id, an employee id that only appears in one office (employee_id_oneoffice), and an employee id that appears in two offices (employee_id_repeats).
What is an assert statement (or other test) I could run to make sure that employees only appear in one office? (i.e that employee_id_oneoffice would pass and employee_id_repeats would fail)

Code:
set seed  339910
clear
set obs 100 
gen office_id = round(_n, 20)
gen employee_id_oneoffice = office_id + floor(runiform(1,4))
gen employee_id_repeats = round(_n, 5)
Please let me know if I can clarify.

Thanks,
Daniel