MongoCursorException: E11000 duplicate key error index

So I'm working on a project where I'm taking a csv file that contains a little more than 100 columns of data by 10,000 rows.  (It's a sample db file -- the final file will be about 200,000,000 rows...) and writing a PHP script to process the csv file into structures that can be inserted as mongodb collections. I'm rocking along and all is working well for initial tests of the algorithm (header + first row of actual data) but when I turn-on processing for the other 9,999 rows, all I get stored into mongo is the first row of data.

I add an echo statement after the insert and I see 10,000 names scroll across my terminal.  So the problem isn't that I'm not getting the data, it's that the data isn't being stored into Mongo.  I try turning on safe writes on my $mongo->insert() function and *bam*, error message:

PHP Fatal error:  Uncaught exception 'MongoCursorException' with message 'E11000 duplicate key error index: insite.testdata.$_id_  dup key: { : ObjectId('4d25fd9a7e03972618000000') }' in /htdocs/framework/parseCSV.php:449
Stack trace:
#0 /htdocs/framework/parseCSV.php(449): MongoCollection->insert(Object(mongorriffic), Array)
#1 {main}
thrown in /htdocs/framework/parseCSV.php on line 449

My code looks like this:

[ccne lang="php" line_numbers="on" tab_size="4" width="100%"] if (($handle = fopen($argv[1], "r")) !== FALSE) { while (($data = fgetcsv($handle, 0, ",")) !== FALSE) { if (!$row) { // use the csv headers to instantiate the new mongo object $objMongo = new mongorriffic($data); } else { // parse the data into the existing structure $objMongo->storeData($data); $mongoCollection->insert($objMongo); } $row++; } fclose($handle); } [/ccne]

I'm calling my internal method (storeDate()) but I'm not creating, populating or resetting an "_id" field value because mongodb is supposed to automagically handle that for me.  What's actually happening is that mongodb  is creating a valid "_id" value for the first record, but since my method does nothing to manipulate the field, the value persists through iterations of the data. I fix this by adding the following line of code after I invoke the insert method:

[ccne lang="php" line_numbers="on" tab_size="4" width="100%"] unset($objMongo->_id); [/ccne]

I re-run my script and *success*!   10,000 records are stored in less than 6-seconds using 4 collections.  BTW,  am *loving* Mongodb....