This issue is a current nuance of the Constituent Relationship area where a User is not able to add the same Organisation with the same 'Relationship Type' which has previously be entered, e.g. Employee/Employer. This is required as a constituent could return to an Organisation later in life where the same Relationship Type could be needed. As a workaround I suggested the User uses a different ‘relationship type’ for the previous entry for the Organisation; Member/Organisation instead of Employee/Employer. This then makes it possible to add the required Organisation.