@Webgr partial answer actually helped me debug this warning @ console log, shame the other part of that answer brought so many downvotes :(
Anyway, here is how I found out what was the cause of this warning in my case:
In my case, another plugin was loading 2 .js libraries after each ajax call, that were absolutely not required nor necessary. Disabling the rogue plugin removed the warning from the log. From that point, you can either try to fix the problem yourself (e.g. limit the loading of the scripts to certain pages or events - this is way too specific for an answer here) or contact 3rd party plugin developer to solve it.
Hope this helps someone.