1. I would be inclined to believe that this is not true. From what I have observed, a Bachelor of Arts and a Bachelor of Politics, Philosophy and Economics do not share a substantial connection, with the former degree being a much more multifaceted degree than the latter. This means that, in the case of the Bachelor of Arts, there is a multitude of fields of study that can be explored, with different universities offering anywhere between ~25 and up to ~70 majors. The Bachelor of Politics, Philosophy and Economics does not offer this and instead focuses specifically on politics, philosophy and economics.
2. This depends on a couple of factors, the first one being your career aspirations, i.e. if you were to study law and PPE, what type of career would you intend to pursue after you graduate? If you wish to pursue a career in law, then the Bachelor of Laws (Honours) is the primary degree that will assist you in finding employment, regardless of the second degree that you study alongside it. The PPE may enable you to complement the knowledge that you gain in the law degree, particularly through its focus on the politics and philosophy components, which could share theoretical connections. Otherwise, the second factor is that a successful start to a career in law depends on whether you achieve favourable results at university (which in turn would indicate a superior level of knowledge), whether you can apply the knowledge and skills that you gained to the profession and whether you have gained employment experience during your time at university (for example, through a clerkship).
3. The Bachelor of Economics includes a larger emphasis on the field of economics than the Bachelor of Politics, Philosophy and Economics which reduces its focus on the field of economics so as to also cover politics and philosophy. A potential quantitative edge may be gained from the Bachelor of Economics given that it is a more pragmatic degree in terms of working in the industry, whereas the PPE degree may provide focus and guidance in another direction/towards a different path. For instance, unless you wish to work in politics, the politics and philosophy elements could provide a foundation of knowledge that would enable you to undertake further related study (such as a Master's degree and/or a PhD) in order to pursue a career in academia.
With that being said, both degrees retain their similarities. More importantly, how effectively you utilise your knowledge and skills to increase your employability and subsequently enhance your career is dependent on you.
4. While you may be able to study additional economics-related subjects as part of the PPE degree (it seems that ANU's program offers this sort of flexibility), it is likely that the Bachelor of Economics will naturally still exceed it in terms of the extent to which it covers economics. The answer therefore depends on how much economics-related education you would consider to be sufficient and whether you would be happy with studying a slightly lesser amount of economics in order to allow for politics and philosophy subjects. In order to assist you in making a more informed decision, it would be a good idea to have a look at the structure of both programs and contact ANU if needed.
5. Double degrees are not necessary in the sense that you can still gain a good quality education and start a rewarding career with a single degree. However, there are definitely benefits to studying a double degree, primarily the fact that gaining knowledge and sets of skills from two or more fields of study increases the number of employment opportunities that you can access because in such a case, you would not be limited to the career opportunities of a single degree. Another benefit is the potential connection between the two degrees, and the resulting ability to combine the knowledge gained from both degrees to support you in your career, such as the potential ability to complement your knowledge of law thanks to your knowledge of politics and philosophy.
On the other hand, a double degree does still take longer to complete than a single degree and is likely to cost more. Of course, if such factors do not constitute an issue for you, then it would probably be a good idea to study a double degree.
I hope this helps!
