Add case for cvar with singleton attr_accessor

Author: ksss

test/rbs/definition_builder_test.rb

@@ -2730,6 +2730,12 @@ class InstanceVariableAttrInstanceVariable
   @instance: Integer
 end
 
+class ClassInstanceVariableSingletonAttrClassInstanceVariable
+  self.@class_instance: Integer
+  attr_accessor self.class_instance: Integer
+  self.@class_instance: Integer
+end
+
 class ClassInstanceVariable
   self.@class_instance: Integer
   self.@class_instance: Integer
@@ -2756,6 +2762,9 @@ class ClassVariable
         assert_raises(RBS::InstanceVariableDuplicationError) do
           builder.build_instance(type_name("::InstanceVariableAttrInstanceVariable"))
         end
+        assert_raises(RBS::ClassInstanceVariableDuplicationError) do
+          builder.build_singleton(type_name("::ClassInstanceVariableSingletonAttrClassInstanceVariable"))
+        end
         assert_raises(RBS::ClassInstanceVariableDuplicationError) do
           builder.build_singleton(type_name("::ClassInstanceVariable"))
         end