About schema : duplicate element definition or reference

[cr]

Could somebody tell me whether the use of duplicate element definition
or reference are permitted in XML schemas?

For example:

<complexType name="PurchaseO rderType">
<sequence>
<element name="shipTo" type="string"/>
<element name="shipTo" type="string"/>
~~~~~~~~~~~~~~~ ~~~~~~~~~~~~~~~ ~~~~~~~~
<element name="billTo" type="ipo:Addre ss"/>
<element ref="ipo:commen t" minOccurs="0"/>
<element ref="ipo:commen t" minOccurs="2"/>
~~~~~~~~~~~~~~~ ~~~~~~~~~~~~~~~ ~~~~~~~~~~~~
<element name="Items" type="ipo:Items "/>
</sequence>
<attribute name="orderDate " type="date"/>
</complexType>

In XMLSpy 5.0 this schema fragment can pass the validation.
The element "shipTo" is duplicate defined, why it can pass the
validation.
And how can I distinguish the element "ipo:commen t" in xml instance?