How do I copy an Advanced Custom Field field group in wordpress?

ACF is made, so there shouldn't be a real reason to copy a group -

If you want the same fields on multiple templates/post types/ categories etc. you can implement them in the "Or add rule group" section -

Here you can have a many as you want.

If you then desire, that only one of the categories(or whatever), should have a specific field, then you take that field out of the rule group, and put it into a new one.

Meaning, everthing that is shared could be combined into one group, and everything that is unique should have it's own.

You can even write custom end points, if the conditions for the rule group doesn't match your needs, but that is more advanced.

Good luck to ya!