Hello,
well, multimap is a C++ STL Feature and GUava in Java. I am not sure what are you using in your backend, but I am pretty sure JS does not have any proper multimap support. So, i guess you would be using multimap from npm, which does not implement multimap how it is supposed to be implemented.
In python, we can still implement multimap using tuples.
We can discuss your data structures in both front end and backend, and can help you out!
Let's talk?