Ethical nature of multiple post-doctoral applications
There is nothing wrong or strange to apply for several fellowships and funding as you describe. The opposite would mean you have to gamble on one and hope it comes through. What can be a bit problematic is perhaps if each proposal causes a lot of work for somebody in a department if you later is a no show despite funding. In such a case it would be good to let people know you are sending in other applications as well so they are not completely in the dark about your situation. I am sure everyone will be sympathetic. If they are not you may not want to go there anyway.
The risk of getting money from several of your applications seems like a luxury problem. Yes, you have to decide which one to go for but that should be a pleasant problem. I would tackle the problem IF it becomes reality and not worry about it at this stage. Your first step is to get applications in, not worry about what to do when and if any funding comes through.
It depends a lot what kinds of grants or fellowships you are applying to. In any case, you have to check the guidelines of the fellowships very throughly. It is not uncommon that you have to indicate if you have submitted a similar proposal elsewhere (stating precisely where). It may even stated that "cross submission" is not allowed.
It is perfectly ethical and reasonable to write multiple applications because chances are low. In most of cases, you will get only one application accepted anyway and it will be no problems. In unlikely case when you get more than one positive response, it should make no problem to pick that suits for you best.
Of course, theoretically if you are good enough for a position A, you should also be good for the positions B, C and D, if the requirements are similar. However this also depends on many random factors. Maybe you know some method they plan to use in research but for some reason did not state clearly in the announcement. Maybe the professor has (grounded or not) personal opinion that your institution or journal where you have published is very bad or very good. People pre-screening the applications may apply different priorities. Depends a lot on who else has applied for the position. Many things can happen.
Waiting for the solution that often takes weeks at least and significantly reduces the number of applications that are possible to write, creating risks that you will never succeed with any.