PHP更新现有MySQL行或插入新行（混淆UPDATE ON DUPLICATE KEY）

I'm trying to code a pretty straightforward updater which can update/create a number of rows in a MySQL table, but I've definitely gone wrong somewhere. Here's what the table, service, looks like:

+---------+----+------------+-------------+--------------------+
| auto_id | id | number     | enlisted    | poe                |
+---------+----+------------+-------------+--------------------+
|      27 | 20 |            | 18.01.1916  | Newcastle, NSW     |
|      28 | 21 | 3088       | 31.07.1915  | Liverpool, NSW     |
|      29 | 21 |            | 19.05.1919  |                    |
|      30 | 22 |            | 10.1916     | 2 MD               |
|      31 | 23 | 1703       | 18.02.1916  | Melbourne          |
|      32 | 24 | 7683       | 13.09.1917  | West Maitland, NSW |
+---------+----+------------+-------------+--------------------+

auto_id is the auto-incrementing primary key. id is the ID of a soldier in another table (all the IDs are unique). As you can see, soldier #21 has two service records. My problem is that in my (ugly, unwieldy) PHP code, when I try and edit a soldier to add a service record, it simply adds new rows, both for the old record (which is being updated) and the new one. Here's the PHP:

for ($i = 0; $i < count($serviceArray); $i++) {
    $currentIDQuery = mysql_query("SELECT id from servicemen WHERE linkname='$target'");
    $currentID = mysql_fetch_row($currentIDQuery);
    $sqlService = "INSERT INTO service ".
                  "(id,number,enlisted,poe,unit,rank,place,casualties,awards,discharged,final_fate,cemetery,memorial)".
                  "VALUES('".$currentID[0]."', '".$serviceArray[$i]['number']."', '".$serviceArray[$i]['enlisted']."', '".$serviceArray[$i]['poe']."', '".$serviceArray[$i]['unit']."', '".$serviceArray[$i]['rank']."', '".$serviceArray[$i]['place']."', '".$serviceArray[$i]['casualties']."', '".$serviceArray[$i]['awards']."', '".$serviceArray[$i]['dis']."', '".$serviceArray[$i]['final']."', '".$serviceArray[$i]['cem']."', '".$serviceArray[$i]['mem']."')".
                  "ON DUPLICATE KEY UPDATE number='".$serviceArray[$i]['number']."', enlisted='".$serviceArray[$i]['enlisted']."', poe='".$serviceArray[$i]['poe']."', unit='".$serviceArray[$i]['unit']."', rank='".$serviceArray[$i]['rank']."', place='".$serviceArray[$i]['place']."', casualties='".$serviceArray[$i]['casualties']."', awards='".$serviceArray[$i]['awards']."', discharged='".$serviceArray[$i]['dis']."', final_fate='".$serviceArray[$i]['final']."', cemetery='".$serviceArray[$i]['cem']."', memorial='".$serviceArray[$i]['mem']."'";
    $retservice = mysql_query( $sqlService, $conn );
    if (!$retservice){
        die('Could not enter service data: ' . mysql_error());
    }
}

As far as I can make out, my problem is that the code will give a new auto_id whatever else it does. The DUPLICATE KEY it needs to be looking at is id, but there can be multiple identical values in there.

For the ON DUPLICATE KEY action to be performed, there would need to be a UNIQUE KEY on the table (in addition to the auto_id column). The INSERT action has to cause a "duplicate key" error.

The UNIQUE KEY could be on a combination of columns, for example, in the sample data, (id,enlisted) appears to be unique. (I'm just guessing at what combination of columns uniquely identify a row.)