PostSelection Formula in order to check for Dual coverage of employees with their contacts

Summary:

Hi I am trying to write a fast formula where I need to check whether the employee is a contact of any another employee in the company and whether have they enrolled them in the same benefit already. if yes, then i should not be able to enroll. Vice versa, if I am trying to enroll my contact (like my spouse or children), it should check whether they have already enrolled themselves and if yes, then it should not allow me to enroll. How can this be done?

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