Import: Column names have duplicates

The exception with the error message you stated is triggered in exactly one place (code slightly truncated for readability):

final public function __construct($source)
{
    $this->_source = $source;

    $this->_init();

    // validate column names consistency
    if (is_array($this->_colNames) && !empty($this->_colNames)) {
        $this->_colQuantity = count($this->_colNames);

        if (count(array_unique($this->_colNames)) != $this->_colQuantity) {
            Mage::throwException(Mage::helper('importexport')->__('Column names have duplicates'));
        }
    }
}

This means, count(array_unique($this->_colNames)) must be unequal to $this->_colQuantity

The $_colNames array is set in Mage_ImportExport_Model_Import_Adapter_Csv::rewind() (which is called during _init().
Once again, the code is slightly truncated for readability:

public function rewind()
{
    // rewind resource, reset column names, read first row as current
    rewind($this->_fileHandler);
    $this->_colNames = fgetcsv($this->_fileHandler, null, $this->_delimiter, $this->_enclosure);
}

The $_delimiter is set to ,, the $_enclosure is set to ".

To try to reproduce the issue, if I copy the CSV extract from your question into a file called test.csv and run the following code:

$f = fopen('test.csv', 'r');
$names = fgetcsv($f, null, ',', '"');    
$qty = count($names);
$uniqueQty = count(array_unique($names));
printf("%d records, %d unique records\n", $qty, $uniqueQty);

it produces the following output:

29 records, 29 unique records

This means, your CSV file is fundamentally okay. The difference must be somewhere else.
Looking at how array_unique works, there is a note how the type handling of array elements changed in PHP 5.2.9: http://php.net/manual/en/function.array-unique.php#refsect1-function.array-unique-changelog

To reproduce the issue, I re-ran my test-script with the SORT_REGULAR option set, but this still gives the same result (which is logical, since reading a file only can give string values).

At this time I believe the difference must be in the CSV file you are using. The Unix and the Windows newline characters (\n and \r\n) are both recognized by the fgetcsv() command, but the old MacOS style newline character (\r) would actually lead to the behavior you are experiencing.

I can't know if that is the reason why you are experiencing the issue, but I suggest you check the CSV file (again). It might also help if you provide a download link somewhere to the unmodified file (no pastebin), so any non-printable characters are preserved.

It might also help if you post the PHP version you are using.