Error when Vundle is loading from multiple sources.

It's quite common to see issues related to Vundle accidentally loading
from several sources.
Try to mitigate this issue by echoing an error.

autoload/vundle.vim

@@ -61,6 +61,20 @@ endif
 " :Plugin command in the vimrc.  It is not possible to do this automatically
 " because when loading the vimrc file no plugins where loaded yet.
 func! vundle#rc(...) abort
+  " If this function has already been called and Vim hasn't finished loading,
+  " that is, we are not sourcing our .vimrc file later on, Vundle is probably
+  " loading from several sources which can lead to unpredictable results.
+  if exists('g:vundle_loading') && g:vundle_loading
+    echoerr 'Vundle is probably loading from several sources. ' .
+          \ 'This can lead to unpredictable results! ' .
+          \ 'Please find and remove the duplication.'
+  endif
+
+  " Set g:vundle_loading only if Vim hasn't finished loading.
+  if !exists('g:vundle_loaded') || !g:vundle_loaded
+    let g:vundle_loading = 1
+  endif
+
   let g:bundle_dir = len(a:000) > 0 ? expand(a:1, 1) : expand('$HOME/.vim/bundle', 1)
   let g:updated_bundles = []
   let g:vundle_log = []
@@ -81,4 +95,12 @@ func! vundle#end(...) abort
   call vundle#config#activate_bundles()
 endf
 
+augroup vundle
+  au!
+
+  " Tell Vundle it has finished loading and Vim is up and running.
+  au VimEnter * unlet g:vundle_loading
+  au VimEnter * let g:vundle_loaded = 1
+augroup END
+
 " vim: set expandtab sts=2 ts=2 sw=2 tw=78 norl: