Description
In my experience, there are few places in data work where problems with data are more evident than when merging datasets -- something that is both a problem (if you think you're doing a one-to-one merge and one of the keys isn't unique in one dataset, you can introduce huge problems) and an opportunity (checking that a merge works as expected is a great way to catch problems).
With that in mind, I'd like to propose adding a check_merge argument that accepts strings one_to_one, one_to_many, may_to_one, and many_to_many. If one of those strings is passed, the merge function would then check uniqueness of merge variables before running the merge, and if for example the merge key for the first dataset has duplicates in a one_to_many merge, it would throw an (informative) exception.
(Stata made a similar move to bake this functionality into its merge command around Stata 12 using 1:1, 1:m, m:1 syntax, which I'm also open to).
Though this functionality can be replicated with user tests, it gets tiring to write them every time...