Skip output check if output not defined on formatter

lib/rspec/core/formatters.rb

@@ -194,10 +194,16 @@ def register(formatter, notifications)
 
     def duplicate_formatter_exists?(new_formatter)
       @formatters.any? do |formatter|
-        formatter.class == new_formatter.class && formatter.output == new_formatter.output
+        formatter.class == new_formatter.class &&
+          has_matching_output?(formatter, new_formatter)
       end
     end
 
+    def has_matching_output?(formatter, new_formatter)
+      return true unless formatter.respond_to?(:output) && new_formatter.respond_to?(:output)
+      formatter.output == new_formatter.output
+    end
+
     def existing_formatter_implements?(notification)
       @reporter.registered_listeners(notification).any?
     end

spec/rspec/core/formatters_spec.rb

@@ -144,9 +144,27 @@ module RSpec::Core::Formatters
             loader.add :documentation, path
           }.to change { loader.formatters.length }
         end
+
+        plain_old_formatter = Class.new do
+          RSpec::Core::Formatters.register self, :example_started
+
+          def initialize(output)
+          end
+        end
+
+        it "handles formatters which do not subclass our formatters" do
+          expect {
+            loader.add plain_old_formatter, output
+          }.to change { loader.formatters.length }
+
+          # deliberate duplicate to ensure we can check for them correctly
+          expect {
+            loader.add plain_old_formatter, output
+          }.to_not change { loader.formatters.length }
+        end
       end
 
-      context "When a custom formatter exists" do
+      context "when a custom formatter exists" do
         specific_formatter = RSpec::Core::Formatters::JsonFormatter
         generic_formatter = specific_formatter.superclass