When you try to open a new REPL buffer in Emacs cider, and you already have one REPL opened, cider will ask you for confirmation, show something like this in Emacs minibuffer

 
An nREPL connection buffer already exists.  Do you really want to create a new one? Yes or no
 

This get annoying if you do it a lot of times. And this makes no sense, why I can have only one REPL buffer? Sometimes I would run a long running task in REPL, for example send a network request, it may takes a while to finish, with only one buffer I can only wait it to finish, if I want to do something else I need a new buffer which connect to the same server, I can do something useful while waiting the task to be done.

Lets disable the confirmation step. First locate the function where the confirmation is called, just do a grep search for popped string, we can find the function "nrepl-check-for-repl-buffer" in nrepl-client.el

 
(defun nrepl-check-for-repl-buffer (endpoint project-directory)
  "Check whether a matching connection buffer already exists.
Looks for buffers where `nrepl-endpoint' matches ENDPOINT,
or `nrepl-project-dir' matches PROJECT-DIRECTORY.
If so ask the user for confirmation."
  (if (cl-find-if
       (lambda (buffer)
         (let ((buffer (get-buffer buffer)))
           (or (and endpoint
                    (equal endpoint
                           (buffer-local-value 'nrepl-endpoint buffer)))
               (and project-directory
                    (equal project-directory
                           (buffer-local-value 'nrepl-project-dir buffer))))))
       (nrepl-connection-buffers))
      (y-or-n-p
       "An nREPL connection buffer already exists.  Do you really want to create a new one? ")
    t))
 

The function y-or-n-p will show a prompt message in the minibuffer and ask for confirmation, if input yes, the function return true, otherwise return false. To disable the confirmation we just let it always return true by replace the function call as t.

 
(autoload 'nrepl-check-for-repl-buffer "cider")
(autoload 'cider-connect "cider")
 
(defun nrepl-check-for-repl-buffer (endpoint project-directory)
  "Check whether a matching connection buffer already exists.
Looks for buffers where `nrepl-endpoint' matches ENDPOINT,
or `nrepl-project-dir' matches PROJECT-DIRECTORY.
If so ask the user for confirmation."
  (if (cl-find-if
       (lambda (buffer)
         (let ((buffer (get-buffer buffer)))
           (or (and endpoint
                    (equal endpoint
                           (buffer-local-value 'nrepl-endpoint buffer)))
               (and project-directory
                    (equal project-directory
                           (buffer-local-value 'nrepl-project-dir buffer))))))
       (nrepl-connection-buffers))
       ;(y-or-n-p
       ;"An nREPL connection buffer already exists.  Do you really want to create a new one? ")
       t
    t))
 

Add the code above to you .emacs file, or just execute it in ielm REPL. Since the code in cider is autoloaded, means its only evaluated when cider-connect is actually called, so we force it load at startup, so our version of the function will take effect.