It looks like you don't get double cruise credits for December sailings. According to Adam's blog re: The December Diamond event, you will get an extra cruise credit. I copied and pasted the following extract from his remarks on the blog:
Link to Adam's blog
"Diamond and Diamond Plus members will get an additional cruise credit for each of the qualifying cruises they complete in December."
There is a link to a list of cruises that qualify.
Wow, DOUBLE cruise credits would be one heck of a perk! On a 12-nter in a JS
, you would have gotten SIX credits!