QR Scan - Detecting Duplicates

I've made a small app that gets a QR code of a scanned item, and then checks it against predefined conditions in the Google Script), and then returns a result, updating the item's row in the Google Sheet if the scanned item fulfills all conditions. As expected, this was pretty slow and there were situation where the item's row wouldn't get updated for whatever reason (Google Script reports weren't useful, as they just report "conflict" or something similar).

Then I've modified the app, so it analyzes all the items against predefined conditions in advance, then stores the results in the TinyDB and uploads them to the Google Sheet when the scanning is completed.

Now this is much faster and I don't notice any scans not being registered. However, now I have another problem. One of the said conditions is to check whether an item has been scanned in the current session. That is not a problem if I scan on a single device, but due to a large number of items, I'd need to scan with multiple devices - and I can't check for duplicates, because I have no way of exchanging data among multiple devices now.

Does anyone have any viable idea on this? Thanks in advance.

Perhaps this might help:

Thanks on the answer! Although it seems kinda resource expensive to get thousands of rows each time?

If you read to the bottom of the howto, there is a method to return only the changed rows....

var changes = [];
 for (var k = 0; k < rng.length; k++) {
  if (JSON.stringify(rng[k]) !== JSON.stringify(cur[k])) { 
 msg =  changes;