Feature request: prevent duplicate relationships

[juresrpcic]

I'm using SuperTable as a User field. The ST acts as a many-to-many relationship between Users (users) and Companies (entries). The relationship has linking fields (user1 has invested X in company1).

SuperTable under User looks like this: Choose company (limit selection to single Entry) // Capital invested (numeric field)

I'd like to check that the user doesn't add a Company twice (prevent duplicate companies).