array (size=4)
  0 => 
    array (size=2)
      'bill_no' => string 'plockbill_202007013' (length=19)
      'deduct_amount' => string '1.00' (length=4)
  1 => 
    array (size=2)
      'bill_no' => string 'plockbill_202007014' (length=19)
      'deduct_amount' => string '3.00' (length=4)
  2 => 
    array (size=2)
      'bill_no' => string 'plockbill_202007013' (length=19)
      'deduct_amount' => string '2.00' (length=4)
  3 => 
    array (size=2)
      'bill_no' => string 'plockbill_202007014' (length=19)
      'deduct_amount' => string '6.00' (length=4)

需要按相同单号（plockbill_202007013，plockbill_202007014）分组后求各自的总金额：

$all_bill_deduct_set = [];
$result =   array();
foreach($all_bill_deduct as $key => $val){
	$result[$val['bill_no']][] = $val['deduct_amount'];
	$all_bill_deduct_set[$val['bill_no']]['all_deduct_amount'] = array_sum($result[$val['bill_no']]);
}

array (size=2)
  'plockbill_202007013' => 
    array (size=1)
      'all_deduct_amount' => float 3
  'plockbill_202007014' => 
    array (size=1)
      'all_deduct_amount' => float 9