There is a group of equivalent schematas as follows, where N is non-negative int:
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema">
  <xs:complexType name="correct.sequence.2">
                        <xs:sequence>
                                    <xs:element name="a" minOccurs="0" />
                                    <xs:element name="b" minOccurs="N" maxOccurs="N" />
                                    <xs:element name="b"/>
                        </xs:sequence>
            </xs:complexType>
</xs:schema>

If N is equal to 1 then schema is valid.
For N from 2 to .... the schema is invalid and Xerces reports:

(2,45): cos-nonambig: b and b (or elements from their substitution group) violate "Unique Particle Attribution". During validation against this schema, ambiguity would be created for those two particles.