Good news: Samsung Galaxy S5 Duos officially gets Marshmallow.
It is an international version: from the UAE from my part.
Here's a screenshot of the About device panel to prove it:
The new baseband is : G900FDXXU1CPB4
As for the build number : MMB29M.G900FDXXU1CPE1
If you're still not migrated to Marshmallow just check for it manually under: Setting, About device, Software update (or equivalent).
Good luck !