I have seen some books published on Amazon and on the main Kindle competitor kobo. Does it violate KDP's terms of service to sell an ebook on Kindle and also distribute the same ebook on a competitor's site?
As NotJohn has pointed out, you only violate Amazon's terms if you enroll in their KDP Select program then list your book elsewhere. KDP Select enrollment is the only way you restrict yourself to Amazon. Don't make that mistake.