These two phrases mean basically the same thing and emphasize that the people have, indeed, turned away from Yahweh. They have disobeyed him and broken the covenant he had made with them. (See: [[rc://en/ta/man/jit/figs-parallelism]])
Yahweh speaks of himself by name to express the certainty of what he is declaring. See how you translated this in [Jeremiah 1:8](../01/08.md). Alternate translation: "this is what Yahweh has declared" or "this is what I, Yahweh, have declared" (See: [[rc://en/ta/man/jit/figs-123person]])