Add guards to prevent double loading of dependencies

Fixes the following warning on Vim startup with late Vundle:

  Vundle error: Name collision for Plugin tpope/vim-repeat.
  Plugin tpope/vim-repeat previously used the name "vim-repeat".
  Skipping Plugin tpope/vim-repeat.
Merge pull request #1 from jasoncodes/vundle-collision

Add guards to prevent double loading of dependencies