Beginning of this month, Citibank announced its 2x miles promotion for International Spends, which came just in time for the summer season. To be able to get 2x miles on your international spends, you do need to go over here and register for the promotion, and then all your international spends between May 1, 2014 and June 30, 2014 will be counted for awarding upto 25000 bonus PM miles. Remember, the deadline to register is May 31, 2014 and you need to receive an acknowledgement from Citibank within 7 days confirming they received your registration and everything is okay.
You can read further details about the promotion in this blogpost. Here are the detailed terms & conditions.