<?php

$Arr1 = array(array ('date' => '2015-11-00', 'mal' => 35),
              array ('date' => '2015-12-00', 'mal' => 20),
              array ('date' => '2016-01-00', 'mal' => 30),
              array ('date' => '2016-01-00', 'mal' => 10),
              array ('date' => '2015-11-00', 'mal' => 50));

$result = array();
foreach ($Arr1 as $data) {
    @$result[$data['date']] += $data['mal'];
}

$newresult = array();
foreach ($result as $date => $mal) {
    $newresult[] = array('date' => $date, 'mal' => $mal);
}

print_r($newresult);