diff --git a/plugins/stoa/controllers/stoa_plugin_controller.rb b/plugins/stoa/controllers/stoa_plugin_controller.rb
index 28d8099..300e38d 100644
--- a/plugins/stoa/controllers/stoa_plugin_controller.rb
+++ b/plugins/stoa/controllers/stoa_plugin_controller.rb
@@ -26,7 +26,7 @@ class StoaPluginController < PublicController
 
   def check_usp_id
     begin
-      render :text => { :exists => StoaPlugin::UspUser.exists?(params[:usp_id]) }.to_json
+      render :text => { :exists => StoaPlugin::UspUser.exists?(params[:usp_id]) && Person.find_by_usp_id(params[:usp_id]).nil? }.to_json
     rescue Exception => exception
       render :text => { :exists => false, :error => {:message => exception.to_s, :backtrace => exception.backtrace} }.to_json
     end
diff --git a/plugins/stoa/lib/ext/person.rb b/plugins/stoa/lib/ext/person.rb
new file mode 100644
index 0000000..816f116
--- /dev/null
+++ b/plugins/stoa/lib/ext/person.rb
@@ -0,0 +1,5 @@
+require_dependency 'person'
+
+class Person
+  validates_uniqueness_of :usp_id
+end
diff --git a/plugins/stoa/lib/stoa_plugin.rb b/plugins/stoa/lib/stoa_plugin.rb
index 9da2b87..9d37d1f 100644
--- a/plugins/stoa/lib/stoa_plugin.rb
+++ b/plugins/stoa/lib/stoa_plugin.rb
@@ -1,4 +1,5 @@
 require_dependency 'person'
+require_dependency 'ext/person'
 
 class StoaPlugin < Noosfero::Plugin
 
diff --git a/plugins/stoa/test/unit/person.rb b/plugins/stoa/test/unit/person.rb
new file mode 100644
index 0000000..f3dbd6e
--- /dev/null
+++ b/plugins/stoa/test/unit/person.rb
@@ -0,0 +1,17 @@
+require File.dirname(__FILE__) + '/../../../../test/test_helper'
+
+class StoaPlugin::Person < ActiveSupport::TestCase
+
+  should 'validates uniqueness of usp_id' do
+    usp_id = 12345678
+    person = create_user('some-person').person
+    person.usp_id = usp_id
+    person.save!
+    another_person = Person.new(:name => "Another person", :identifier => 'another-person', :usp_id => usp_id)
+
+    assert !another_person.valid?
+    assert another_person.errors.invalid?(:usp_id)
+  end
+
+end
+
--
libgit2 0.21.2