doctor: add warning for dual private configs

bin/doom-doctor

@@ -125,6 +125,16 @@
      (concat "\nMacOS users should use homebrew (https://brew.sh) to install Emacs\n"
              "  brew install emacs --with-modules --with-imagemagick --with-cocoa"))))
 
+(section! "test-private-config")
+(let ((xdg-dir (concat (or (abbreviate-file-name (getenv "XDG_CONFIG_HOME"))
+                           "~/.config")
+                       "/doom/"))
+      (doom-dir "~/.doom.d/"))
+  (when (and (file-directory-p xdg-dir)
+             (file-directory-p doom-dir))
+    (warn! "Detected two private configs, in %s and %s" xdg-dir doom-dir)
+    (explain! "The second directory will be ignored, as it has lower precedence.")))
+
 
 ;; --- is the environment set up properly? --------------------