I have the following HTML:
For me there were no duplicates in my code. This happend as a result of me duplicating a module to get a headstart on the new one, and then doing a module wide find/replace and changing the names of the files.
Even though there was no duplicate anymore, and I stopped and started the browsersync based server, the error continued.
Resolving it was done by removing the .tmp directory that the build system was creating for the dev environment.
Evidently the FountainJS generator I'm using creates a build system that leaves the .tmp directory dirty in some cases like this. It's caught me a few times now.