Add warnings for duplicate attributes and relationship names

lib/jsonapi/resource.rb

@@ -404,6 +404,8 @@ def attribute(attr, options = {})
           ActiveSupport::Deprecation.warn('Id without format is no longer supported. Please remove ids from attributes, or specify a format.')
         end
 
+        check_duplicate_attribute_name(attr) if options[:format].nil?
+
         @_attributes ||= {}
         @_attributes[attr] = options
         define_method attr do
@@ -879,6 +881,8 @@ def _add_relationship(klass, *attrs)
 
           check_reserved_relationship_name(relationship_name)
 
+          check_duplicate_relationship_name(relationship_name)
+
           # Initialize from an ActiveRecord model's properties
           if _model_class && _model_class.ancestors.collect{|ancestor| ancestor.name}.include?('ActiveRecord::Base')
             model_association = _model_class.reflect_on_association(relationship_name)
@@ -1009,6 +1013,18 @@ def check_reserved_relationship_name(name)
           warn "[NAME COLLISION] `#{name}` is a reserved relationship name in #{_resource_name_from_type(_type)}."
         end
       end
+
+      def check_duplicate_relationship_name(name)
+        if _relationships.include?(name.to_sym)
+          warn "[DUPLICATE RELATIONSHIP] `#{name}` has already been defined in #{_resource_name_from_type(_type)}."
+        end
+      end
+
+      def check_duplicate_attribute_name(name)
+        if _attributes.include?(name.to_sym)
+          warn "[DUPLICATE ATTRIBUTE] `#{name}` has already been defined in #{_resource_name_from_type(_type)}."
+        end
+      end
     end
   end
 end

test/fixtures/active_record.rb

@@ -1501,8 +1501,6 @@ class LineItemResource < V6::LineItemResource; end
 
     class CustomerResource < V6::CustomerResource
       model_name 'Api::V7::Customer'
-      attribute :name
-      has_many :purchase_orders
     end
 
     class ClientResource < JSONAPI::Resource

test/unit/resource/resource_test.rb

@@ -202,6 +202,22 @@ def test_class_relationships
     assert_equal(relationships.size, 2)
   end
 
+  def test_duplicate_relationship_name
+    assert_output nil, "[DUPLICATE RELATIONSHIP] `mother` has already been defined in CatResource.\n" do
+      CatResource.instance_eval do
+        has_one :mother, class_name: 'Cat'
+      end
+    end
+  end
+
+  def test_duplicate_attribute_name
+    assert_output nil, "[DUPLICATE ATTRIBUTE] `name` has already been defined in CatResource.\n" do
+      CatResource.instance_eval do
+        attribute :name
+      end
+    end
+  end
+
   def test_find_with_customized_base_records
     author = Person.find(1)
     posts = ArticleResource.find([], context: author).map(&:_model)