ON DUPLICATE KEY UPDATE - Don't Have A Key But Want To Update

This may seem like more of a PHP question but I will come onto the MySQL part in a minute.

Basically I have a PHP script that lets users upload 2 different CSV files to different tables. The problem I have is that sometimes these CSV files are updated and then need to be reimported into the database through the script. I have made this script totally generic so that when you change the table that is being entered to the rest of the script will alter itself to import the correct CSV to that table. But a problem arises when I have a CSV file that contains each product sold and the shopper who bought the product (so the shopperID may appear in more than 1 row) - however this file does not have a unique id for each row meaning that when I use an INSERT INTO with a ON DUPLICATE KEY UPDATE... it will just insert more rows instead of updating rows because there is now key in the table.

The format of the table is like this (with some example data to show you more clearly):

+-----------+-------------+---------+------+--------+------+
| ShopperID | Product Code | Product | Price | Amount | Units |
+-----------+-------------+---------+------+--------+------+
546733 1315 prod 1 64.00 64.00 0
546733 1316 prod 2 43.99 43.99 0

OK just ignore the last 2 fields - that is a problem with the old system there was which does not add the units up right.

Anyway basically I might have the same shopper ordering 2 things which will come up on different rows - I have 3 things I cannot do:

1. I can't make the ShopperID field the PRIMARY KEY because I have many times where there are 2 rows with the same shopper
2. And I cannot just do an UPDATE when it gets to a duplicate entry because it will overwrite every time it gets to a shopper that already exists.
3. I don't want to add a unique primary key to the CSV as this will mean that the system I am creating loses its ability of being automatic because the person (an admin) that uploads the file will have to make sure that it has that extra field which is auto incrementing.

Does anyone have any ideas on how I can get round this?

Here is my PHP function which contains the query:
PHP

// this function for insert data to csv
function makeINSERTS($text, $table, $tablefields, $correctcsv){
    $insert = array(); //make array for hold data insert
    $i = 0;
    $success = true;
    
    while(list($key, $val) = each($text)){
        // Insert the data
        $insert[$i] = "INSERT INTO ".$table."  VALUES('";
        $insert[$i] .= implode("','", $val);
        $insert[$i] .= "') ON DUPLICATE KEY UPDATE ";
        foreach($tablefields as $k=>$field){
            $insert[$i] .= "`$field`='{$val[$k]}',";
        }
        $insert[$i] = substr($insert[$i],0,strlen($insert[$i])-1);
        // echo $insert[$i].'<p></p>'
        $result = mysql_query($insert[$i]);
        if(!$result) {
            echo 'FAILURE to insert/update any or all of the database form uploaded CSV!<br />'
            $success = false;
            die('Query failed: ' . mysql_error().'<br />');
        }
        $i++;
    }
    if($success == true && $correctcsv == true){
        echo 'SUCCESS in uploading the CSV file to the database!<br />'
    }
    else {
    }
return $insert;
}