PHP Yet another Multidimensional Associative Array problem

I know this is right in front of me, but I need to add an element to each row of an array..

I have this array:

Array (
    [0] => Array (
         [Elements] => values
    )
    [1] => Array (
        [Elements] => values
    )
)

Now, I want to add an element to the end of each one that holds the file name of the originating data.

The part of code this takes place in, is in a class method to look for duplicates already in the database. If it is a duplicate, we add the $fileData iteration to the $duplicates array which gets returned to the calling function. It basically looks like this:

while($data = fgetcsv($handle)) {           
    $leadDataLine = array_combine($headers, $data);

    // Some data formatting on $leadDataLine not important for this question...

    // Add the line to the stack
        $leadData[] = $leadDataLine;
    //array_push($leadData, $leadDataLine);
    unset($leadDataLine);       
} 
 $dup[] = $lead->process($leadData);

The lead class:

<?php

public function process(&$fileData) {
    $duplicates = array();
    // Process the information

    foreach($fileData as $row) {
            // If not a duplicate add to the database
            if (!$this->isDuplicate($row)) {
                // Add the lead to the database.
                $this->add($row); 
            } else {
                // is a duplicate, add to $dup

                $duplicates[] = array("Elements" => $row['Values']);


                                    /* 
                                     * Here is where I want to add the file name to the end of $duplicates
                                     * This has to be here because this class handles different sources of data,
                                     * Not all data will have a FileName key
                                     */
                if (array_key_exists("FileName", $row))
                    $duplicates["FileName"] =  $row["FileName"];
                    // array_push($duplicates, $row["FileName"]);

            }

    }
    print_r($duplicates);
    return $duplicates;

}

What happens with the code I have, or using array_push:

Array (
    [0] => Array (
        [Elements] => values
    )
    [FileName] => correct_file.csv
    [1] => Array (
        [Elements] => Values
    )
) 

Notice, it is not on element 1..

What am I doing wrong here.