Sorry to be the guy who necros an old post like that but as of September 2016 the issue still persists. I am running ACF 5.4.4, WPML 3.5.1.1 and WordPress 4.6.1 so pretty much the bleeding edge on all fronts. I am experiencing the exact same bug as described above – when duplicating a field group from the main language to another I get a separate translation even though I check the Duplicate tickbox. This can lead to having a hard to diagnose discrepancy between field group definitions in different languages especially when working with a team of people. The groups need to be duplicates to remain up to date with the main language.
