'git branch' when origin branch with same name exists

hi,

After helping someone on IRC who got themselves into a bit of a problem, 
I have a feature request for git.

In this case, the user wanted to checkout a branch of a module.  They did:

   git clone git://git.gnome.org/gtk+
   cd gtk+
   git branch gtk-3-4

Obviously this is a user error, but it's a pretty innocent one, and puts 
the user in a bad state.  When they figure they should have typed "git 
checkout gtk-3-4" it is already too late -- they will be taken onto 
their locally-created copy of the master branch.

So feature request: 'git branch' should not allow creating a local 
branch that has the same name as a branch that already exists on 
'origin' (or any remote?) without some sort of --force flag.  It could 
instead helpfully say:

   """

     The branch you are trying to create already exists on the origin.
     Are you sure you don't want to checkout this branch using
     'git checkout gtk-3-4'

  """

or something like that.

Thoughts?