diff --git a/app/models/user.rb b/app/models/user.rb
index 868207697..99eda6475 100644
--- a/app/models/user.rb
+++ b/app/models/user.rb
@@ -68,7 +68,7 @@ class User < Principal
   MAIL_LENGTH_LIMIT = 60
 
   validates_presence_of :login, :firstname, :lastname, :mail, :if => Proc.new { |user| !user.is_a?(AnonymousUser) }
-  validates_uniqueness_of :login, :if => Proc.new { |user| !user.login.blank? }, :case_sensitive => false
+  validates_uniqueness_of :login, :if => Proc.new { |user| user.login_changed? && user.login.present? }, :case_sensitive => false
   validates_uniqueness_of :mail, :if => Proc.new { |user| !user.mail.blank? }, :case_sensitive => false
   # Login must contain lettres, numbers, underscores only
   validates_format_of :login, :with => /^[a-z0-9_\-@\.]*$/i
diff --git a/test/unit/user_test.rb b/test/unit/user_test.rb
index a78cb3c70..23692bfdf 100644
--- a/test/unit/user_test.rb
+++ b/test/unit/user_test.rb
@@ -134,6 +134,20 @@ class UserTest < ActiveSupport::TestCase
     assert_equal "john", @admin.login
   end
 
+  def test_update_should_not_fail_for_legacy_user_with_different_case_logins
+    u1 = User.new(:firstname => "new", :lastname => "user", :mail => "newuser1@somenet.foo")
+    u1.login = 'newuser1'
+    assert u1.save
+
+    u2 = User.new(:firstname => "new", :lastname => "user", :mail => "newuser2@somenet.foo")
+    u2.login = 'newuser1'
+    assert u2.save(false)
+
+    user = User.find(u2.id)
+    user.firstname = "firstname"
+    assert user.save, "Save failed"
+  end
+
   def test_destroy_should_delete_members_and_roles
     members = Member.find_all_by_user_id(2)
     ms = members.size