Warn if the same gem (same version) is added twice

 Creates a warning about duplicating a gem with the same version
 & adds a test to spec/install/gems/simple_case_spec.rb

  modified:   lib/bundler/dsl.rb
  modified:   spec/install/gems/simple_case_spec.rb
Merge pull request #2679 from jendiamond/same_gem_twice_boo

warn if the same gem (same version) is added twice