list-invalid.xml

<?xml version="1.0" encoding="UTF-8"?>
<plan>
  <workbooks/>
  <users>
    <mappings>
      <userMappingx>
        <source mapping="user-mapping-source"/>
        <destination mapping="user-mapping-destination"/>
      </userMappingx>
      <groupMapping>
        <source mapping="user-mapping-source"/>
        <destination mapping="user-mapping-destination"/>
      </groupMapping>
      <userMapping>
        <source mapping="user-mapping-source"/>
        <destination mapping="user-mapping-destination"/>
      </userMapping>
    </mappings>
  </users>
</plan>

list.xml

<?xml version="1.0" encoding="UTF-8"?>
<plan>
  <workbooks/>
  <users>
    <mappings>
      <userMapping>
        <source mapping="user-mapping-source"/>
        <destination mapping="user-mapping-destination"/>
      </userMapping>
      <groupMapping>
        <source mapping="user-mapping-source"/>
        <destination mapping="user-mapping-destination"/>
      </groupMapping>
      <userMapping>
        <source mapping="user-mapping-source"/>
        <destination mapping="user-mapping-destination"/>
      </userMapping>
    </mappings>
  </users>
</plan>

list.xsd

<?xml version="1.0" encoding="utf-8"?>
<!-- Created with Liquid Technologies Online Tools 1.0 (https://www.liquid-technologies.com) -->
<xs:schema attributeFormDefault="unqualified" elementFormDefault="qualified" xmlns:xs="http://www.w3.org/2001/XMLSchema">
  <xs:element name="plan">
    <xs:complexType>
      <xs:sequence>
        <xs:element name="workbooks" />
        <xs:element name="users">
          <xs:complexType>
            <xs:sequence>
              <xs:element name="mappings">
                <xs:complexType>
                  <xs:sequence>
                    <xs:choice maxOccurs="unbounded">
                      <xs:element name="userMapping">
                        <xs:complexType>
                          <xs:sequence>
                            <xs:element name="source">
                              <xs:complexType>
                                <xs:attribute name="mapping" type="xs:string" use="required" />
                              </xs:complexType>
                            </xs:element>
                            <xs:element name="destination">
                              <xs:complexType>
                                <xs:attribute name="mapping" type="xs:string" use="required" />
                              </xs:complexType>
                            </xs:element>
                          </xs:sequence>
                        </xs:complexType>
                      </xs:element>
                      <xs:element name="groupMapping">
                        <xs:complexType>
                          <xs:sequence>
                            <xs:element name="source">
                              <xs:complexType>
                                <xs:attribute name="mapping" type="xs:string" use="required" />
                              </xs:complexType>
                            </xs:element>
                            <xs:element name="destination">
                              <xs:complexType>
                                <xs:attribute name="mapping" type="xs:string" use="required" />
                              </xs:complexType>
                            </xs:element>
                          </xs:sequence>
                        </xs:complexType>
                      </xs:element>
                    </xs:choice>
                  </xs:sequence>
                </xs:complexType>
              </xs:element>
            </xs:sequence>
          </xs:complexType>
        </xs:element>
      </xs:sequence>
    </xs:complexType>
  </xs:element>
</xs:schema>