Edit: I forgot, have you tried setting the advertiser to non-connectable? That way you should be able to get duplicate scan results
I am dealing with a similar issue, that is, reliably track the RSSI values of multiple advertising devices over time.
It is sad, the most reliable way i found is not nice, rather dirty and battery consuming. It seems due to the number of android devices that handle BLE differently the most reliable.
I start LE scan, as soon as i get a callback i set a flag to stop and start scan again. That way you work around that DUPLICATE_PACKET filter issue since it resets whenever you start a fresh scan.
The ScanResults i dump into a sqlite db wich i shrink and evaluate once every x seconds.
It should be easy to adapt the shrinking to your use case, i.e. removing entries that are older than X, and then query for existance of a device to find out if you received a ScanResult in the last X seconds. However dont put that X value too low, as you must take into account that you still lose alot of advertisement packets on android LE scan, compared to a BLE scan on i.e. bluez..
Edit:
I can add some information i already found for speeding up the performance on Advertisement discovery. It involves modifying and compiling the bluedroid sources and root access to the device. Easiest would be building a full android yourself, i.e. Cyanogenmod.
When a LE scan is running, the bluetooth module sends the scan sesponse via HCI to the bluedroid stack. There various checks are done until it finally gets handed to the Java onScanResult(...)
which is accessed via JNI.
By comparing the log of the hci data sent from the bluetooth module (can be enabled in /etc/bluetooth/bt_stack.conf) with debug output in the bluedroid stack aswell as the Java side i noticed that alot of advertisement packets are discarded, especially in some check. i dont really understand, beside that it has something to do with the bluedroid inquiry database
From the documentation of ScanResult we see that the ScanRecord includes the advertisement data plus the scan response data. So it might be that android blocks the report until it got the scan response data/ until it is clear there is no scan response data. This i could not verify, however a possibility.
As i am only interested in rapid updates on the RSSI of those packets, i simply commented that check out. It seems that way every single packet i get from the bluetooth moduly by hci is handed through to the Java side.
In file btm_ble_gap.c in function BOOLEAN btm_ble_update_inq_result(tINQ_DB_ENT *p_i, UINT8 addr_type, UINT8 evt_type, UINT8 *p)
comment out to_report = FALSE;
in the following check starting on line 2265.
/* active scan, always wait until get scan_rsp to report the result */
if ((btm_cb.ble_ctr_cb.inq_var.scan_type == BTM_BLE_SCAN_MODE_ACTI &&
(evt_type == BTM_BLE_CONNECT_EVT || evt_type == BTM_BLE_DISCOVER_EVT)))
{
BTM_TRACE_DEBUG("btm_ble_update_inq_result scan_rsp=false, to_report=false,\
scan_type_active=%d", btm_cb.ble_ctr_cb.inq_var.scan_type);
p_i->scan_rsp = FALSE;
// to_report = FALSE; // to_report is initialized as TRUE, so we basically leave it to report it anyways.
}
else
p_i->scan_rsp = TRUE;