Add helpful warning message for shared example groups

I found myself with a strange warning message not long ago saying the following:
```
WARNING: Shared example group 'a flagger_or_tagger' has been previously defined
at:
  /Users/devoncestes/esh/IRT/spec/services/shared_examples/flagger_or_tagger_spec.rb:10
...and you are now defining it at:
  /Users/devoncestes/esh/IRT/spec/services/shared_examples/flagger_or_tagger_spec.rb:10
The new definition will overwrite the original one.
```

Turns out that this was happening because the file in which we defined our
shared example group ended in `_spec.rb`, which triggered the Rspec auto
loading, thus creating the strange warning. I had no idea how ths could happen
until I opened up an issue and @myronmarston pointed out the error. I figured
that a helpful warning message that could guide the user to fixing this warning
themselves might be good to add, so I've done so here!

Author: devonestes

lib/rspec/core/shared_example_group.rb

@@ -213,16 +213,35 @@ def valid_name?(candidate)
 
         def warn_if_key_taken(context, key, new_block)
           existing_module = shared_example_groups[context][key]
-
           return unless existing_module
 
-          RSpec.warn_with <<-WARNING.gsub(/^ +\|/, ''), :call_site => nil
-            |WARNING: Shared example group '#{key}' has been previously defined at:
-            |  #{formatted_location existing_module.definition}
-            |...and you are now defining it at:
-            |  #{formatted_location new_block}
-            |The new definition will overwrite the original one.
-          WARNING
+          old_definition = formatted_location existing_module.definition
+          new_definition = formatted_location new_block
+
+          if old_definition == new_definition
+            RSpec.warn_with <<-WARNING.gsub(/^ +\|/, ''), :call_site => nil
+              |WARNING: Shared example group '#{key}' has been previously defined at:
+              |  #{old_definition}
+              |...and you are now defining it at:
+              |  #{new_definition}
+              |The new definition will overwrite the original one.
+              |
+              |You might be seeing this warning because of a naming issue
+              |If you have defined a shared example group in a file that
+              |ends in _spec.rb, and you're also including that file in another
+              |spec, causing the shared example group to be loaded twice.
+              |You can solve this by renaming your file so it doesn't end in
+              |_spec.rb and continuing to include the file manually.
+            WARNING
+          else
+            RSpec.warn_with <<-WARNING.gsub(/^ +\|/, ''), :call_site => nil
+              |WARNING: Shared example group '#{key}' has been previously defined at:
+              |  #{old_definition}
+              |...and you are now defining it at:
+              |  #{new_definition}
+              |The new definition will overwrite the original one.
+            WARNING
+          end
         end
 
         def formatted_location(block)

spec/rspec/core/shared_example_group_spec.rb

@@ -69,7 +69,7 @@ def find_implementation_block(registry, scope, name)
             expect(Kernel).to_not respond_to(shared_method_name)
           end
 
-          it "displays a warning when adding a second shared example group with the same name" do
+          it 'displays a warning when adding a second shared example group with the same name' do
             group.send(shared_method_name, 'some shared group') {}
             original_declaration = [__FILE__, __LINE__ - 1].join(':')
 
@@ -82,6 +82,21 @@ def find_implementation_block(registry, scope, name)
             expect(warning).to_not include 'Called from'
           end
 
+          it 'displays a helpful message when you define a shared example group in *_spec.rb file' do
+            warning = nil
+            allow(::Kernel).to receive(:warn) { |msg| warning = msg }
+            declaration = nil
+
+            2.times do
+              group.send(shared_method_name, 'some shared group') {}
+              declaration = [__FILE__, __LINE__ - 1].join(':')
+            end
+
+            better_error = 'You might be seeing this warning because of a naming issue'
+            expect(warning).to include('some shared group', declaration, better_error)
+            expect(warning).to_not include 'Called from'
+          end
+
           it 'works with top level defined examples in modules' do
             expect(RSpec::configuration.reporter).to_not receive(:deprecation)
             RSpec.describe('example group') { include_context 'top level in module' }