Raise an error if write_subscription is called twice

lib/graphql/schema/subscription.rb

@@ -173,8 +173,12 @@ def self.topic_for(arguments:, field:, scope:)
       # later in execution.
       # @return [void]
       def write_subscription
-        @subscription_written = true
-        context.schema.subscriptions.write_subscription(context.query, [event])
+        if subscription_written?
+          raise GraphQL::Error, "`write_subscription` was called but `#{self.class}#subscription_written?` is already true. Remove a call to `write subscription`."
+        else
+          @subscription_written = true
+          context.schema.subscriptions.write_subscription(context.query, [event])
+        end
         nil
       end
 

spec/graphql/schema/subscription_spec.rb

@@ -722,7 +722,14 @@ def subscribe
 
       class DirectWrite < ImplicitWrite
         type String
+        def subscribe
+          write_subscription
+          super
+        end
+      end
 
+      class DirectWriteTwice < DirectWrite
+        type String
         def subscribe
           write_subscription
           super
@@ -732,6 +739,8 @@ def subscribe
       class Subscription < GraphQL::Schema::Object
         field :direct, subscription: DirectWrite
         field :implicit, subscription: ImplicitWrite
+        field :direct_twice, subscription: DirectWriteTwice
+        field :with_payload, subscription: DirectWriteWithPayload
       end
 
       use WriteCheckSubscriptions.new
@@ -751,5 +760,12 @@ class Subscription < GraphQL::Schema::Object
       assert_equal [1], res.context[:written_events].map(&:size)
       assert_equal false, res.context.namespace(:subscriptions)[:subscriptions].values.first.subscription_written?
     end
+
+    it "raises if write_subscription is called twice" do
+      err = assert_raises GraphQL::Error do
+        DirectWriteSchema.execute("subscription { directTwice }")
+      end
+      assert_equal "`write_subscription` was called but `DirectWriteSchema::DirectWriteTwice#subscription_written?` is already true. Remove a call to `write subscription`.", err.message
+    end
   end
 end