found another prettier way

app/models/group.rb

@@ -37,11 +37,13 @@ class Group < Principal
 
   acts_as_customizable
 
-  validates_presence_of :lastname
-  validates_uniqueness_of :lastname, :case_sensitive => false
-  validates_length_of :lastname, :maximum => 30
 
   before_destroy :remove_references_before_destroy
+  
+  alias_attribute(:groupname, :lastname)
+  validates_presence_of :groupname
+  validate :uniqueness_of_groupname
+  validates_length_of :groupname, :maximum => 30
 
   def to_s
     lastname.to_s
@@ -106,4 +108,8 @@ class Group < Principal
     Journal::WorkPackageJournal.update_all({ :assigned_to_id => deleted_user.id },
                                            { :assigned_to_id => id })
   end
+  
+  def uniqueness_of_groupname
+    errors.add :groupname, :taken if Group.find_by_lastname(lastname)
+  end
 end

config/initializers/10-patches.rb

@@ -75,13 +75,6 @@ module ActiveModel
                 })
               end
             end
-          elsif attribute == :lastname && @base.class.to_s == 'Group'
-            attr_name = @base.class.human_attribute_name(:groupname, :default => 'Groupname')
-            full_messages << I18n.t(:"errors.format", {
-              :default   => "%{attribute} %{message}",
-              :attribute => attr_name,
-              :message   => message
-            })
           else
             attr_name = attribute.to_s.gsub('.', '_').humanize
             attr_name = @base.class.human_attribute_name(attribute, :default => attr_name)