Haha I had this same question earlier, a few threads down, and I have been doing my research!
Apparently Kathrinidad is right about it being a hard hatch, but I have read that it IS doable! There was a twin egg successfully hatched in the UK. Made the news. And a small farmer somewhere in the US midwest claims to have hatched one, but it is not recorded. Double chicken eggs are hatched frequently. Just like those chicken eggs, a double duck egg would need help to hatch if you could bring them "full term." Sometimes you have to make air holes, etc.
I am sooooo trying next season if my hen keeps laying doubles.
*edited because my grammar was atrocious*