Fix Bundler/OrderedGems autocorrect behavior

  ## Summary

  This PR improves `Bundler/OrderedGems` autocorrect  behavior on the
  specific  situation.
  When group includes duplicate gems, and one of duplicate gems is
  in the first line of group, and one of them has
  offense `Bundler/OrderedGems`.
  The cop replace the gem out of group and the gem in group.

  ### Before

  For example, the target Gemfile is like below.

  ```ruby
  gem 'a'

  group :development do
    gem 'b'
    gem 'c'
    gem 'b'
  end
  ```

  ### Expected

  Expected result of `rubocop -a --only Bundler/OrderedGems`
  is like below.

  ```ruby
  gem 'a'

  group :development do
    gem 'b'
    gem 'b'
    gem 'c'
  end
  ```

  ### Actual

  But actual result is like below.

  ```ruby
  gem 'b'

  group :development do
    gem 'a'
    gem 'b'
    gem 'c'
  end
  ```

CHANGELOG.md

@@ -2,6 +2,10 @@
 
 ## master (unreleased)
 
+### Bug fixes
+
+* [#5623](https://github.com/bbatsov/rubocop/pull/5623): Fix `Bundler/OrderedGems` when a group includes duplicate gems. ([@colorbox][])
+
 ## 0.53.0 (2018-03-05)
 
 ### New features
@@ -3225,3 +3229,4 @@
 [@satyap]: https://github.com/satyap
 [@unkmas]: https://github.com/unkmas
 [@elebow]: https://github.com/elebow
+[@colorbox]: https://github.com/colorbox

lib/rubocop/cop/bundler/ordered_gems.rb

@@ -58,7 +58,7 @@ def autocorrect(node)
 
         def previous_declaration(node)
           declarations = gem_declarations(processed_source.ast)
-          node_index = declarations.find_index(node)
+          node_index = declarations.map(&:location).find_index(node.location)
           declarations.to_a[node_index - 1]
         end
 

spec/rubocop/cop/bundler/ordered_gems_spec.rb

@@ -276,4 +276,42 @@
       RUBY
     end
   end
+
+  context 'When there are duplicated gems in group' do
+    let(:source) { <<-RUBY.strip_indent }
+      gem 'a'
+
+      group :development do
+        gem 'b'
+        gem 'c'
+        gem 'b'
+      end
+    RUBY
+
+    it 'registers an offense' do
+      expect_offense(<<-RUBY.strip_indent)
+        gem 'a'
+
+        group :development do
+          gem 'b'
+          gem 'c'
+          gem 'b'
+          ^^^^^^^ #{format(message, 'b', 'c')}
+        end
+      RUBY
+    end
+
+    it 'autocorrects' do
+      new_source = autocorrect_source_with_loop(source)
+      expect(new_source).to eq(<<-RUBY.strip_indent)
+        gem 'a'
+
+        group :development do
+          gem 'b'
+          gem 'b'
+          gem 'c'
+        end
+      RUBY
+    end
+  end
 end