Thanks, I have done a few tests recently and can confirm you can just re-upload local JSON export over the top of the existing fields and all is fine.
My worry wasn’t about duplication but loss of data within pages/posts etc using the fields – in case by overwriting (best way to describe the task) data was lost for some reason.
Either way, if you have the local > online environment you can happily just upload a new JSON each time you alter field groups and it will keep everything as it was with the addition of your new work.
