Coming in 3.3.0 is a new config item for matching devices called match_ip_no_data. This will be set to 'y' by default. This will be the last match item to be tested.
The idea behind match_ip_no_data is that if you are discovering a remote subnet and either have devices without management protocols, or devices that you don't have credentials for, you don't want duplicated devices.
By default the match_ip rule is set to 'n' because of DHCP. When you can a device on a remote subnet, the IP and DNS Hostname might be the only items that are returned. Neither of these are considered unique.
As a result, you often end up with duplicate devices.
match_ip_no_data hopes to address this.
If you have a result as above with precious little information, we will match on a device in the system table with the same IP, but only if it has not been audited (ie, it has an unknown or unclassified type and/or no serial number).
It's like a "match of last resort".
Hopefully this will prevent the duplicate devices from appearing.
Really though - you should have credentials for devices you are discovering 
This code (along with the new config item) will be included in 3.3.0.
To enable this (albeit without the config item to disable it), edit the file:
Linux
/usr/local/open-audit/code_igniter/application/models/m_devices.php
Windows
c:\xampp\open-audit\code_igniter\application\models\m_devices.php
And add the code below starting at line 1271.
It should come after the match_hostname section and before the line.
$temp = @(string)$details->id;
# check IP Address in system table for a device with no other data
if ((empty($match->match_ip_no_data) or strtolower($match->match_ip_no_data) == 'y') and empty($details->id) and !empty($details->ip) and filter_var($details->ip, FILTER_VALIDATE_IP)) {
# Check the system table for an ip match on a device without a type or serial
if (empty($details->id)) {
$sql = "SELECT system.id FROM system WHERE system.ip = ? AND system.ip NOT LIKE '127%' AND system.ip NOT LIKE '1::%' AND system.status != 'deleted' and (system.type = 'unknown' or system.type = 'unclassified') and system.serial = ''";
$sql = $this->clean_sql($sql);
$data = array(ip_address_to_db($details->ip));
$query = $this->db->query($sql, $data);
$row = $query->row();
if (count($row) > 0) {
$details->id = $row->id;
$log->system_id = $details->id;
$message = new stdClass();
$message->message = 'HIT on IP Address No Data (system table).';
$message->command_status = 'success';
$message->command_output = 'IP: ' . $details->ip . ', SystemID : ' . $details->id;
$log_message[] = $message;
foreach ($log_message as $message) {
$log->message = $message->message;
$log->command_status = $message->command_status;
$log->command_output = $message->command_output;
discovery_log($log);
}
$message->command_output = '';
return $details->id;
}
}
$message = new stdClass();
$message->message = 'MISS on IP Address No Data.';
$message->command_status = 'notice';
$message->command_output = 'IP: ' . $details->ip;
$log_message[] = $message;
} else {
if (strtolower($match->match_ip) != 'y') {
$message = new stdClass();
$message->message = 'Not running match_ip_no_data, matching rule set to: ' . $match->match_ip . '.';
$message->command_status = 'notice';
$message->command_output = '';
$log_message[] = $message;
} else if (!empty($details->id)) {
$message = new stdClass();
$message->message = 'Not running match_ip_no_data, device id already set';
$message->command_status = 'notice';
$message->command_output = '';
$log_message[] = $message;
} else if (empty($details->ip)) {
$message = new stdClass();
$message->message = 'Not running match_ip_no_data, ip not set.';
$message->command_status = 'notice';
$message->command_output = '';
$log_message[] = $message;
} else {
$message = new stdClass();
$message->message = 'Not running match_ip_no_data.';
$message->command_status = 'notice';
$message->command_output = '';
$log_message[] = $message;
}
}