Help wanted on how to implement custom business logic

[olia-bn]

We have an entity UserProduct that stores connection between User and Product (and some extra information).
Customer is using API extensively and would like not having to check for duplicates before poking an API endpoint (to know whether to create or update); ideally he'd like to submit User and Product and let the app figure it out.

I was planning on implementing a custom action controller, but documentation is clear tht in most cases, custom controllers are strongly discouraged.

What would be a good way to implement such feature in line with best practices?