Duplicate log output from log4cl in multi-threading

I got this weird issue that log4cl output duplicate logs in thread functions when running as script:

#!/usr/bin/sbcl --script

(require :asdf)
(require :log4cl)

(log:config :debug)

(log:info "begin")

(defun worker ()
  (let ((name (sb-thread:thread-name sb-thread:*current-thread*)))
    (log:info "~a started" name)
    (log:debug "~a ~a" name (sb-ext:get-time-of-day))
    (log:info "~a stoped" name)))

(loop for thread in (loop for i below 2
                          collect (sb-thread:make-thread
                                   'worker
                                   :name (format nil "worker-~a" i)
                                   :arguments nil))
      do (sb-thread:join-thread thread))

(log:info "done")

Output:

$ ./foo.lisp
 <INFO> [17:40:25] cl-user foo.lisp () - begin
 <INFO> [17:40:25] cl-user foo.lisp (worker) - worker-0 started
 <INFO> [17:40:25] cl-user foo.lisp (worker) - worker-0 started
 <INFO> [17:40:25] cl-user foo.lisp (worker) - worker-1 started
 <INFO> [17:40:25] cl-user foo.lisp (worker) - worker-1 started
<DEBUG> [17:40:25] cl-user foo.lisp (worker) - worker-0 1713346825
<DEBUG> [17:40:25] cl-user foo.lisp (worker) - worker-0 1713346825
<DEBUG> [17:40:25] cl-user foo.lisp (worker) - worker-1 1713346825
<DEBUG> [17:40:25] cl-user foo.lisp (worker) - worker-1 1713346825
 <INFO> [17:40:25] cl-user foo.lisp (worker) - worker-0 stoped
 <INFO> [17:40:25] cl-user foo.lisp (worker) - worker-0 stoped
 <INFO> [17:40:25] cl-user foo.lisp (worker) - worker-1 stoped
 <INFO> [17:40:25] cl-user foo.lisp (worker) - worker-1 stoped
 <INFO> [17:40:25] cl-user foo.lisp () - done

I'm using SBCL 2.4.1, and log4cl is the latest from quicklisp. I also tested log4cl from https://github.com/sharplispers/log4cl and got the same issue.

It appears that the log duplication only occurs in thread functions.

However, if load the file into SBCL REPL, there is no duplication:

sbcl --noinform --no-userinit
* (load "foo.lisp")
 <INFO> [17:48:36] cl-user foo.lisp () - begin
 <INFO> [17:48:36] cl-user foo.lisp (worker) - worker-0 started
 <INFO> [17:48:36] cl-user foo.lisp (worker) - worker-1 started
<DEBUG> [17:48:36] cl-user foo.lisp (worker) - worker-1 1713347316
 <INFO> [17:48:36] cl-user foo.lisp (worker) - worker-1 stoped
<DEBUG> [17:48:36] cl-user foo.lisp (worker) - worker-0 1713347316
 <INFO> [17:48:36] cl-user foo.lisp (worker) - worker-0 stoped
 <INFO> [17:48:36] cl-user foo.lisp () - done
T

Could someone tell me is it a bug of log4cl, or am I missing something?