function addSafely( collection, element, database )
	{
		// 1. IE doesn't support customData on text nodes;
		// 2. Text nodes never get chance to appear twice;
		if ( !element.is || !element.getCustomData( 'block_processed' ) )
		{
			element.is && CKEDITOR.dom.element.setMarker( database, element, 'block_processed', true );
			collection.push( element );
		}
	}
Add to collection with DUP examination.
@param {Object} collection
@param {Object} element
@param {Object} database