Do not extend a Model more than one time

* This prevents clobbering of properties and other ivars set in the
  model when extended
* Check for the existence of previously named properties in a
  cleaner/simpler way

lib/dm-core/model.rb

@@ -113,6 +113,8 @@ def self.extra_extensions
     # TODO: document
     # @api private
     def self.extended(model)
+      return if Model.descendants.include?(model)
+
       unless model.ancestors.include?(Resource)
         model.send(:include, Resource)
       end

lib/dm-core/property_set.rb

@@ -16,12 +16,14 @@ def [](name)
       @properties[name]
     end
 
+    alias super_slice []=
+
     # TODO: document
     # @api semipublic
     def []=(name, property)
       if named?(name)
         add_property(property)
-        map! { |p| p == property || p.name == property.name ? property : p }
+        super_slice(index(property), property)
       else
         self << property
       end
@@ -42,8 +44,12 @@ def values_at(*names)
     # TODO: document
     # @api semipublic
     def <<(property)
-      add_property(property)
-      super
+      if named?(property.name)
+        super_slice(index(property), property)
+      else
+        add_property(property)
+        super
+      end
     end
 
     # TODO: document