Although I'm sure nothing would make Amazon happier then to have you purchase each of your books multiple times you don't have to. When you create your Kindle account you can then use that same account on any other device that supports the Amazon Kindle software including your Mac, PC, Windows phone, IOS phone, or android phone.
All you need to do is sign into the Kindle software on those devices with the same account and then download those same books you've already purchased to that device. Beware however that your location in the book is stored for each device so if you have multiple people trying to be the same book that's not quite as legal and you'll be sharing one location within the book. On the other hand you can what you like with your multiple devices. Just beware that there's only one current location for each book on the Amazon Kindle service.