Golang uses := to represent declaration and simultaneous assignment to at least one new variable. = only works if the LHS variables have already been declared. If you use :=, at least one of the variables on the LHS has to be new. So the following snippet would be an error:

  a := 3
  ...
  a := 4

This is valid:

  a := 3
  ...
  a = 4

As is this:

  a := 3
  ...
  a, b := 4, 2

And this is an error again:

  a := 3
  b := 4
  a, b := 1, 2

Since neither a nor b are new variables.

As someone who has never used go, the third example feels counter-intuitive to me. I would expect a, b := 4, 2; to be equivalent to a := 4; b := 2; is there a reason it was done this way?

It is as you say, however, a and b had already been declared, which is the error.

But... why the latter?

Because if you're modifying a function and you're adding a new variable to hold some intermediary value, you might accidentally pick a variable name that already exists. If you're in the habit of writing := to signal your intent to create a new variable, that error will probably get caught at compile time rather than being a source of subtle bugs where you trample data needed by something else.

The multi-variable case is trickier, especially because (result, err) is such a common pattern and you neither want to impede the very common case of storing a result into a new variable nor to require a profusion of differently-named variables for errors that are very likely to just get folded anyway. So it allows it if any of the variables are new rather than requiring all. But clearing a large chunk of the bug surface is still much better than not.

(This is only my extrapolated reasoning, of course.)

Oh, this is a good time to remember Prolog exists. Assignment isn't even a concept there, but equality can act on so many different ways!