Fixed error range for duplicate attribute.

[angelozerr]

If we have duplicate prefix (see following sample with duplicate xmlns:tns) :

<?xml version="1.0" encoding="UTF-8" ?>
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema" 
	xmlns:tns="http://camel.apache.org/schema/spring"
	xmlns:tns="http://camel.apache.org/schema/spring" version="1.0">

The error range highlight the last attribute (in this case version)

although it should highlight the first or second xmlns:ts.

Problem comes from XMLSyntaxErrorCode with AttributeNSNotUnique

Please note there are the same problem with attribute without NS (AttributeNotUnique):

<a attr="" attr="" attr2="" />

Today attr2 is higlighted although it should be attr which should be highligted.

To fix this 2 problems, we must use arguments which contains the attribute name to highlight by using XMLPositionUtility.selectAttributeNameFromGivenNameAt I think.

[angelozerr]

Fixed with #456