Advanced Custom Fields lets you move a field to another field group, but moving removes it from where it was. There is no built-in way to copy a field into another group and keep the original.
Ranabikram Field Copy for ACF adds exactly that. A new Copy link appears in each field’s action row, right next to Edit, Duplicate, Move and Delete. Click it, pick a destination field group, and the field is copied into that group while the original stays put.
The copy reuses ACF’s own duplication routine, so it:
This plugin runs entirely inside your WordPress admin and makes no external requests — no data leaves your site.
Note: like ACF’s own Move and Duplicate, this copies the field definition, not the saved post values.