Graph.add: type already defined

[mikeshulman]

Using the version of merlin that's installed with OCaml 5.3.0 (I guess 5.4.1-503), I sometimes get an error message in Emacs that says

merlin-mode error: Graph.add: type already defined

and merlin stops working entirely for that buffer. I would love to post a MWE but I have no idea what might be causing this; it generally only happens with fairly large files, and removing small pieces of them at random tends to make the error go away. For example, I get the error in the file bin/olorin.ml in this branch. If you can tell me how to give more useful information to help debug the issue, I would be happy to try it.