Did you know that there are dice nerds? Okay, you probably did... if you haven't heard of the infamous zocchihedron, or seen the recent The New Yorker article on the d120, you've probably at least seen enough Dungeons & Dragons dice sets to know that they come in all shapes and colors. But maybe you didn't know how far things have gone...
We are all familiar with the standard d6, the traditional six sided die, a simple cube. You may or may not be aware of the d12, a dodecahedron with 12 sides. You probably did not know that people were passing aesthetic judgments on these dice, but they were. The d12 rolls better, with smaller sides and more obtuse vertices. The d6 is chunky and clumsy, resulting in fewer side-changes per roll, reducing (theoretically) the randomization of your roll.
The obvious solution is to map the d6 onto the d12. Instead of numbering your dodecahedron 1-12, you number it 1-6, and then 1-6 again. Statistically it functions the same as a d6, but now dice nerds will pay more for it.
There is a downside, of course. The traditional dice bag of variegated polyhedral dice made it easy to tell which die was which visually or by feel. On the other hand, having to locate a d12 versus a doublesix in a mixed bag takes a non-trivial inspection... which is why a true die fan will have them in separately labeled compartments.
The doublesix is also sometimes referred to as a 2xd6. As you may have deduced, there are also triplefour dice, although apparently no quadruplethree dice.