This issue is linked to the issue http://issues.ez.no/IssueView.php?Id=14751&.

When a workflow like addlocation is started, the process key is generated but the parameters array are not taken in count. So it is impossible to add a seconde location in two different workflow processes.

With the proposed patch the issue is fixed :
eZWorflowProcess::createKey() : line 170 to 183

static function createKey( $parameters )
{
/* Patch */
$string = '';
foreach ( array_keys( $parameters ) as $key )
{
$value = $parameters[$key];
if ( is_array( $value ) )
{
$value = serialize( $value );
}
$string .= $key . $value;
}
return md5( $string );
/* End of Patch */
}