An implementation of "completing-read-multiple" by using "completing-read" in a loop. This allows it to use "completing-read-function" for multiple completion as well. For example, if you have "ido-ubiquitous-mode" enabled, you get multiple completion via ido.

crm-via-cr.el

(defun my-crm-custom (prompt table &optional predicate require-match initial-input hist def inherit-input-method)
  (loop with return-list = nil
        with next-value = nil
        with def-list = (s-split crm-separator (or def ""))
        with def-list-no-empty-string = (remove "" def-list)
        with def-text = (when def-list-no-empty-string
                          (concat "(" (s-join crm-separator def-list) ")"))
        ;; Need to clear this
        with def = nil
        ;; Save original prompt and construct prompt with defaults
        with orig-prompt = prompt
        with prompt = (concat orig-prompt def-text)
        ;; Enable entry of empty string with ido
        with ido-ubiquitous-enable-old-style-default = t
        ;; Pre-expand completions table
        with table = (delete-dups
                      (nconc def-list-no-empty-string
                             (all-completions "" table predicate)))
        with predicate = nil
        do (message "Table: %S" table)
        do (setq next-value
                 (completing-read prompt
                                  table
                                  predicate
                                  require-match
                                  initial-input
                                  hist
                                  nil   ; Default is handled elsewhere
                                  inherit-input-method))
        ;; Fold empty string to nil
        if (string= next-value "")
        do (setq next-value nil)
        ;; Empty input on first prompt returns result
        if (null next-value)
        return (or return-list def-list)
        else
        collect next-value into return-list
        and do (message "Read next value: %S" next-value)
        ;; Remove selected item from stuff, and unset initial things
        and do (setq
                prompt (concat orig-prompt "("
                               (s-join crm-separator return-list) crm-separator)
                table (delete next-value table)
                initial-input nil)))
                
;; Test it out
(my-crm-custom "Select several: " '("A" "B" "C" "D" "E") nil t nil nil "D,E,A")