This system will always work for playing a double. Now what I say does not mean it will always work every single time the situation comes up, but still no matter what it is a good system to keep in mind.
Look back to the last 7 drawings, mid and eve. What digit is missing completely from all those draws. That's the double you play. Some recommend playing the cold digit, i.e. the future double, with the current hottest digit. But it does not mean that that will always happen. It really could be any digit that pairs with the double.
I have always used 7 draws, some people use more some people use less. 7 is just my basis for looking at things. So in the last 7 draws you don't see any 6's. Start thinking about playing 66. It's really as simple as that. That's the first way I started thinking about doubles, and I've seen it enough to remember about it and think about it. Cold digits always like to pop up, and sometimes they pop up twice at the same time.