The double-D rule is simply: Differences are Difficult

When designing interfaces, it's important to remember that inconsistencies create challenges for users. If our users have already used a feature in our application, such as a left side navigation, don't suddenly move the navigation to the right side of the page. It's one more point of cognitive load the user will have to absorb - even if it's the exact same menu, just shifted, it's still forcing the user to think - and thus taking up some useful processing that would be best applied to newer/different concepts. Conclusively, don't create differences; rather, be consistent.