They are not equal. Most universities in the world don't have a double degree program, i.e. you will receive 2 qualifications (e.g. B.S/B.A.) when you finish it. I know Australia and probably New Zealand has these, and they usually take 1 yr extra than just 1 bachelor (4 yrs instead of 3 yrs, for double degrees in engineering, 5 yrs instead of a normal 4 yrs for a single degrees.) http://sydney.edu.au/courses/study_area/science-and-technology I don't know any other countries where you can do double degrees in one university. In some European countries, like France, Germany, some university provides these degrees with another university, and they are only for top students who can run around places and complete 2 degrees in the same time (3 yrs) as a normal student, the programs are probably very structured. Here's an example: http://www.upmc.fr/fr/formations/di...ce/double_cursus_sciences_et_philosophie.html
It's in french but with a little google translate you can probably understand it. This is a double degree in philosophy and science. As you can see, the structure is pretty set and you don't have much choices.
Double majors are common in U.S. and most other European countries but you receive one degree by the end of it.
What do you mean by "double degree" and "double major"? Different schools define things differently.
Actually, lots of people in the US earn two bachelors degrees. Lots of students meet the requirements for two different degrees and then earn two bachelors degrees. e.g. a BSc in Physics and a BSc in Mathematics. It is also common in the US to participate in a 3+2 program where a student spends 3 years at one school earning a bachelors degree in science at a liberal arts school and then goes to a more technical school for 2 years to earn a bachelors degree in engineering.
At my university, a double major is when both majors are within the same college(math and physics for example) and a double degree is when you the two majors you have are in different colleges(liberal arts and engineering for example).
Double degrees and double majors aren't all that uncommon in Canada. I'm not so sure that either one is better than the other -- I think they usually take about the same amount of time to do (5 years).
As has already been mentioned, the difference, at least at my school, is just based on whether the two majors are in the same college. One isn't inherently better than the other, it just depends on what you want to do with it. For instance, if I wanted to go into astrophysics, I'd be best served having a double major in Physics/Astronomy. If I wanted to go into economics, maybe I'd go for a dual degree in Math/Economics. It's entirely relative to what your career goals are.
So generally, the farther two disciplines are from each other, the more likely you'd get a dual degree. The more closely related they are, the more likely you'll have a double major. Dual degrees may take a little longer to get, depending on the subjects, but I know many people who've done it in the usual 4 years.
In Australia, a normal degree only takes 3 yrs, regardless of how many majors you choose to do in it. If it's a single degree, it's 3 yrs. Unless it's a professional degree (doctors, vets, pharm, etc) or if it's engineering (4 yrs).
Double degrees programs usually last for 4 yrs, and they don't need to relate to each other at all. A lot of times they are in different departments, and really they are treated as two degrees and you can do more than one major in each if you choose to. How do you have time? If you look at a degree in Australia and a degree in America, there's no obligation to learn anything extra other than what your degree is supposed to deliver, no US history, professional practice, etc, if you do physics, you just do physics. That's why degrees are shortened to 3 yrs. Of course you can still pick other things you like to do in your electives. But in a double degrees program, you don't get many electives.