An associative array in PHP can only have one entry per key. So when you create a PHP array like this:
array(
"identification.WKN" => "555750",
"identification.ISIN" => "DE005557504",
"identification.YAHOO" => "DTE.DE",
"identification.YAHOO" => "DTEGY"
)
the second time you use the key "identification.YAHOO"
it replaces the first value in the array.
Fortunately, the $addToSet operator in MongoDB can be combined with the $each-operator which allows you to pass a whole array of values for a key, not just a single one. My PHP is a bit rusty, but this should work:
array(
"identification.WKN" => "555750",
"identification.ISIN" => "DE005557504",
"identification.YAHOO" => array(
"$each" => array("DTE.DE", "DTEGY")
)
)