Hi, I'm having the same issue.
My Magic Fields write panel has two groups -- one containing regular fields and the other being a group of fields that can be duplicated (so you can "Add another" and a whole new set of the same fields is available).
When I use the Duplicate Post plugin, it duplicates the regular MF fields just fine. But it doesn't duplicate the multiple sets of fields from that second group. For instance, I have 7 instances of that second group of fields and I don't get any instances duplicated.
When I use mlajo's function, it copies on instance of that group -- but not all 7. So one step forward... but not quite enough!
Would love to know if anyone has managed to resolve this issue...