Bug 121003 Data source property binding error
When there are two data source property binding, the second binding cannot be set.The exception will be reported: org.eclipse.birt.report.model.api.metadata.PropertyValueException: The value "odaDataSource" already exists. at org.eclipse.birt.report.model.api.validators.StructureListValidator.doCheckStructureList(StructureListValidator.java:223) at org.eclipse.birt.report.model.api.validators.StructureListValidator.validateForAdding(StructureListValidator.java:96) at org.eclipse.birt.report.model.core.DesignElement.checkStructureList(DesignElement.java:2642) at org.eclipse.birt.report.model.command.PropertyCommand.addItem(PropertyCommand.java:444) at org.eclipse.birt.report.model.api.DesignElementHandle.setPropertyBinding(DesignElementHandle.java:2086) at org.eclipse.birt.report.designer.internal.ui.dialogs.DataSourcePropertyBindingPage.performOk(DataSourcePropertyBindingPage.java:192)
model/org.eclipse.birt.report.model/src/org/eclipse/birt/report/model/api/validators/StructureListValidator.java
*/
	
	public final static String NAME = "StructureListValidator"; 
	private static StructureListValidator instance = new StructureListValidator( );
	/**
	 * Returns the singleton validator instance.
	 * 
	 * @return the validator instance
	 */
	public static StructureListValidator getInstance( )
	{
		return instance;
	}
	/**
	 * Validates whether a new structure can be added to structure list.
	 * 
	 * @param element
	 *            the element holding the structure list
	 * @param propDefn
	 *            definition of the list property
	 * @param list
	 *            the structure list
	 * @param toAdd
	 *            the structure to add
	 * @return error list, each of which is the instance of
	 *         <code>SemanticException</code>.
	 */
	public List validateForAdding( DesignElementHandle element,
			IPropertyDefn propDefn, List list, IStructure toAdd )
	{