Hello Eli,

thanks for looking into this! 

Eli Zaretskii <eliz@gnu.org> writes:
> The duplicate use of with-current-buffer is sub-optimal,
> IMO.  What about the simpler code below:
>
>   (when sieve-manage-log
>     (let* ((existing-log-buffer (get-buffer sieve-manage-log))
>            (log-buffer (or existing-log-buffer
>                            (get-buffer-create sieve-manage-log))))
>       (with-current-buffer log-buffer
>         (unless existing-log-buffer
>           ;; Do this only once, when creating the log buffer.
>           (set-buffer-multibyte nil)
>           (buffer-disable-undo))
>         (goto-char (point-max))
>         (apply #'insert args)))))

Yes, that provides more insight into what the code intends to do. Here's
the patch (with additional updated doc string):