Bug 122854 There is redundant columns in "available columns" list in "export data" dialog.
Description: There is redundant columns in "available columns" list in "export data" dialog. Steps to reproduce: 1. Preview attached design in web viewer. 2. Select the table and click on "export" button. 3. There is a "detail" column name in "available columns" list which is not data set column.
engine/org.eclipse.birt.report.engine/src/org/eclipse/birt/report/engine/api/impl/DataExtractionTask.java
currentResult = new ExtractionResults( subIter, 
						selectedColumns, getScriptExpressions (query.getRowExpressions( )) );
				return currentResult;
			}
			catch( BirtException be )
			{
				be.printStackTrace( );
			}
		}
		return null;
	}
	
	/*
	 * check if the selected columns is valid, if no column is selected, then initialize the
	 * selected column using row expression.
	 */
	private void validateSelectedColumns( IBaseQueryDefinition query )
			throws EngineException
	{
		assert query != null;
		Collection exprs = getScriptExpressions ( query.getRowExpressions( ) );
		if ( selectedColumns != null )
		{
			for ( int i = 0; i < selectedColumns.length; i++ )
			{
				boolean findColumn = false;
				Iterator iter = exprs.iterator( );
				while ( iter.hasNext( ) )
				{
					IScriptExpression expr = (IScriptExpression) iter.next( );