How to prevent dual enrollment of the Child Dependents if both the parents are Employees of the comp

Content

My Client is offering Employer paid Medical Insurance with no cap on the Dependents for all of their employers. There are cases where both the spouses are Employees of the company.

How can we ensure that the child dependents arent enrolled twice in the Benefit Plan and sent to the Benefit carriers? 

Actually they want a validation to throw if the second parent tries to enroll after the first parent had completed the Dependent enrollment. I want to know if its possible through config/fast formula?