name conflict

i have a problem with name conflict, i do not understand if this schema document is valid or not:
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema" elementFormDefault="qualified" attributeFormDefault="unqualified">
    <xs:element name="pippo">
        <xs:complexType>
            <xs:sequence>
                <xs:element name="d">
                    <xs:complexType>
                    </xs:complexType> 
                </xs:element>
                <xs:element name="a" type="xs:string"/>
                <xs:choice> 
                    <xs:element name="b" type="xs:string"/> 
                    <xs:element name="a" type="xs:byte"/> <!-- is it legal ? -->
                    <xs:element name="a" type="xs:byte"/> 
                </xs:choice>
                <xs:sequence>
                    <xs:element name="a"> <!-- and this ? -->
                         <xs:complexType>
                            
                         </xs:complexType>   
                </xs:sequence>
            </xs:sequence>
        </xs:complexType>
    </xs:element>
</xs:schema>

thanks for your help
andrea boriero