Prevent adding version option with flags already in use

lib/command.js

@@ -501,18 +501,32 @@ Expecting one of '${allowedValues.join("', '")}'`);
   }
 
   /**
-   * Add an option.
+   * Check for option flag conflicts.
+   * Register option if no conflicts found.
+   * Throw otherwise.
    *
    * @param {Option} option
-   * @return {Command} `this` command for chaining
+   * @api private
    */
-  addOption(option) {
+
+  _registerOption(option) {
     const matchingOption = (option.short && this._findOption(option.short)) ||
       (option.long && this._findOption(option.long));
     if (matchingOption) {
       throw new Error(`Cannot add option '${option.flags}'${this._name && ` to '${this._name}'`} since an option using same flag has already been added
 - conflicts with option '${matchingOption.flags}'`);
     }
+    this.options.push(option);
+  }
+
+  /**
+   * Add an option.
+   *
+   * @param {Option} option
+   * @return {Command} `this` command for chaining
+   */
+  addOption(option) {
+    this._registerOption(option);
 
     const oname = option.name();
     const name = option.attributeName();
@@ -528,9 +542,6 @@ Expecting one of '${allowedValues.join("', '")}'`);
       this.setOptionValueWithSource(name, option.defaultValue, 'default');
     }
 
-    // register the option
-    this.options.push(option);
-
     // handler for cli and env supplied values
     const handleOptionValue = (val, invalidValueMessage, valueSource) => {
       // val is null for optional option used without an optional-argument.
@@ -1827,7 +1838,7 @@ Expecting one of '${allowedValues.join("', '")}'`);
     description = description || 'output the version number';
     const versionOption = this.createOption(flags, description);
     this._versionOptionName = versionOption.attributeName();
-    this.options.push(versionOption);
+    this._registerOption(versionOption);
     this.on('option:' + versionOption.name(), () => {
       this._outputConfiguration.writeOut(`${str}\n`);
       this._exit(0, 'commander.version', str);