Hi Julius,
I'm not too familiar with the details of lab.js but I suspect this doesn't have to do with the server side, but instead with how the experimental scripts themselves are written.
Are you sure that lab.js sends data more than once? In general, because each time you send data to the server you have internet traffic and put load on the server, we don't recommend that data are sent after every single trial (or in your case, response). But it might be worth sending data after every (say) 5th or 10th response, depending on how important each item is.
Essentially, you should look at your code (or as Felix, the developer from lab.js) to see if this behaviour is expected and how to change it.
Hope this helps
Best
Elisa