don't implement same interface twice in SDL output

Author: dleavitt

lib/graphql/language/printer.rb

@@ -170,7 +170,7 @@ def print_object_type_definition(object_type)
       end
 
       def print_implements(type)
-        " implements #{type.interfaces.map(&:name).join(" & ")}"
+        " implements #{type.interfaces.map(&:name).uniq.join(" & ")}"
       end
 
       def print_input_value_definition(input_value)

spec/graphql/schema/build_from_definition_spec.rb

@@ -556,6 +556,36 @@ def assert_schema_and_compare_output(definition)
       assert_schema_and_compare_output(schema)
     end
 
+    it "only adds the interface to the type once" do
+      schema = <<-SCHEMA
+interface Named implements Node {
+  id: ID
+  name: String
+}
+
+interface Node {
+  id: ID
+}
+
+type Query {
+  thing: Thing
+}
+
+type Thing implements Named & Node & Timestamped {
+  id: ID
+  name: String
+  timestamp: String
+}
+
+interface Timestamped implements Node {
+  id: ID
+  timestamp: String
+}
+      SCHEMA
+
+      assert_schema_and_compare_output(schema)
+    end
+
     it 'supports simple output enum' do
       schema = <<-SCHEMA
 schema {