Description:
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The package.xml file (version 2.0) does not validate against the current schema (http://pear.php.net/dtd/package-2.0.xsd) if you have more than one <release> child in the <changelog> section. This is also the case with the example file on http://pear.php.net/manual/en/guide.developers.package2.php so I guess the problem lies in the schema.

Test script:
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Just try the demo package file on http://pear.php.net/manual/en/guide.developers.package2.php. 

Possible solution:

I'm not an expert with XSD and I have not tested this, but I guess line 409:

<xs:element name="release">

should be changed to:

<xs:element name="release" minOccurs="1" maxOccurs="unbounded">

In the xsd on http://pear.php.net/dtd/package-2.0.xsd.

Expected result:
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No validation errors

Actual result:
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Validation error on the second <release> element in the <changelog> section (line 220 in the example file).