Whenever you want to add a new user you must make sure that the uidNumber is set to a unique number, if you have many users it could sometimes take a while to find which is the last uidNumber set to a user, to find the last uidNumber run this.
ldapsearch -H ldaps://your-ldap-domain -D "cn=Manager,dc=domain,dc=com" -W | awk '/uidNumber: / {print $2}' | sort | tail -n 1