When using the Basic Editor (19.11.07)- clicking the Tag Repeated option in Firefox, the screen will reload (sort of) but the field will not be duplicated. If that same tag repeated option is chosen a second time, Koha will then create a second tag.
When I test this in master and 19.11.07 using Firefox 78.0.2 on macOS I don't see the problem. The tag repeats without refreshing the page. If you're seeing the page refresh that suggests there might be a JavaScript error. Is there anything in the console? Some custom JavaScript interfering perhaps?
I also tested this and could not replicate the problem. Closing WORKSFORME, please re-open if you can still replicate the behaviour in maintained versions.