PDOException: SQLSTATE[23000]: Integrity constraint violation: 1062 Duplicate entry '' for key 'uri': INSERT INTO {file_managed}

Below log message appears when i am trying to upload three files using the cck node adding programatically. not sure as to what mistake i am doing below is the code

PDOException: SQLSTATE[23000]: Integrity constraint violation: 1062 Duplicate entry '' for key 'uri': INSERT INTO {file_managed} (filesize, status, timestamp) VALUES (:db_insert_placeholder_0, :db_insert_placeholder_1, :db_insert_placeholder_2); Array ( [:db_insert_placeholder_0] => 0 [:db_insert_placeholder_1] => 1 [:db_insert_placeholder_2] => 1309100206 ) in drupal_write_record() (line 6851 of /opt/lampstack-3-0-6-0/apps/drupal/htdocs/includes/common.inc).

$validators = array(
		'file_validate_extensions' => array('html htm'),
	);
	$pdfvalidators = array(
		'file_validate_extensions' => array('pdf'),
	);
	$imgvalidators = array(
		'file_validate_extensions' => array('gif jpg jpeg png'),
	);
		/*
		Check to see what fields are getting saved on node save
		$sampleNode = node_load('125');
		echo "<PRE>" . print_r($sampleNode, TRUE) . "</pre>";
		die;*/
    if($file = file_save_upload('table_upload', $validators,'', FILE_EXISTS_RENAME))
    {
		
		$chart_file = file_save_upload('image_upload', $imgvalidators,'public://images', FILE_EXISTS_RENAME);
		$pdf_file = file_save_upload('pdf_upload', $pdfvalidators,'public://pdf', FILE_EXISTS_RENAME);
		$node = new stdClass();
		$node->type = 'event';
		node_object_prepare($node);
		$node->title    = $form_state['values']['title'];
		$node->field_second_line['und'][0]['value'] = $form_state['values']['second_line'];
		$node->testevent['und'][0]['value'] = $form_state['values']['event_number'];
		$node->language = LANGUAGE_NONE;
		$mime = 'application/xhtml+xml';
		$pdf_mime = 'application/pdf';
		$chart_mime = 'image/jpeg';
		$node->testevent['und'][0] = array(
				array(
					'fid' => $file->fid,
					'filename' => $file->filename,
					'uri' => $file->uri,
					'filesize' => $file->filesize,
					'mimetype' => $mime,
					'status' => 1,
				),
			);

		$node->field_pdf['und'][0] = array(
				array(
					'fid' => $pdf_file->fid,
					'filename' => $pdf_file->filename,
					'uri' => $pdf_file->uri,
					'filesize' => $pdf_file->filesize,
					'mimetype' => $pdf_mime,
					'status' => 1,
				),
			);

		$node->field_chart['und'][0] = array(
				array(
					'fid' => $chart_file->fid,
					'filename' => $chart_file->filename,
					'uri' => $chart_file->uri,
					'filesize' => $chart_file->filesize,
					'mimetype' => $chart_mime,
					'status' => 1,
					'data'=>array('alt' => 'Excel alt',
						'title' => 'Excel File',
					),
				),
			);
	  $node->body[$node->language][0]['summary'] = 'this is test';
	  $node->body[$node->language][0]['format']  = 'full_html';
	 
	  $path = 'content/'.$file->filename.'_'. date('YmdHis');
	  $node->path = array('alias' => $path);
	  node_save($node);