There seems to be a bug in the documentation: https://docs.mongodb.com/manual/tutorial/configure-x509-client-authentication/#client-x-509-certificate

A client x.509 certificate’s subject, which contains the Distinguished Name (DN), must differ from that of a Member x.509 Certificate. Specifically, the subjects must differ with regards to at least one of the following attributes: Organization (O), the Organizational Unit (OU) or the Domain Component (DC)

While the first part is correct, as the DN must differ. The field which is usually different is the CN (Common Name) which isn't included in the list of attributes. Therefore, this is misleading and incorrect.