I think you should double space it since the essay is uploaded. If you are copying and pasting it on to the site, then you would not use hard returns - no hitting the enter key in the essay because when applying for Columbia they specifically wrote that. But I'm not really sure about the common application. It should be the same. Just double space, indent, etc to be safe. Or maybe ask the admissions directly, its the best way to make sure...
Hope it goes well for your application.