These two phrases mean basically the same thing and emphasize that the people have, indeed, forsaken Yahweh. (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]])