How to make tags children unique?

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How to prohibit duplicate tag in my restriction tag in my xml file?for example in my xml file I have two locale tag,but it should be only one tagthis is my xml file :\[code\] <app:string name="firstName"> <app:restriction> <app:regex>^\w*$</app:regex> <app:type/> <app:locale/> <app:locale/> </app:restriction> </app:string>\[/code\]and this is my xsd for string tag:\[code\]<xs:element name="string"> <xs:complexType> <xs:complexContent> <xs:extension base="main:BaseType"> <xs:sequence> <xs:element name="restriction" type="main:StringRestriction" minOccurs="0" maxOccurs="1"/> </xs:sequence> <xs:attribute name="lang" type="main:LocaleTypes"/> </xs:extension> </xs:complexContent> </xs:complexType></xs:element><xs:complexType name="BaseType"> <xs:attribute name="name" type="main:nameType" use="required"/> <xs:attribute name="readonly" type="xs:boolean" use="optional" default="true"/></xs:complexType><xs:complexType name="StringRestriction"> <xs:complexContent> <xs:extension base="main:RestrictionBase"> <xs:sequence> <xs:choice maxOccurs="1"> <xs:element type="xs:string" name="locale"/> <xs:element type="xs:string" name="type"/> <xs:element type="xs:string" name="regex"/> <xs:element type="xs:string" name="maxLen"/> <xs:element type="xs:string" name="minLen"/> </xs:choice> </xs:sequence> </xs:extension> </xs:complexContent></xs:complexType>\[/code\]