Description:
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When using $node->set_attribute on a previously existing attribute with a namespace, the attribute gets doubled rather then updated - interestingly enough, a second update does update the "doubled" attribute just fine...

Additionally, $node->remove_attribute('test:attrib'); doesn't work with the previously existing attribute either while it removes the newly doubled attribute.

Reproduce code:
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$dom=domxml_open_mem('<?xml version="1.0" ?><foo:root xmlns:test="http://test.de/xml" xmlns:foo="http://test.de/foo" test:attrib="0" />');

 $root = $dom->document_element();  
 $root->set_attribute('test:attrib','1');
 $root->set_attribute('test:attrib','2');
 
 echo $dom->dump_mem();

Expected result:
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<?xml version="1.0"?>
<foo:root xmlns:test="http://test.de/xml" xmlns:foo="http://test.de/foo" test:attrib="2"/>

Actual result:
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<?xml version="1.0"?>
<foo:root xmlns:test="http://test.de/xml" xmlns:foo="http://test.de/foo" test:attrib="0" test:attrib="2"/>