diff --git a/app/models/membership.rb b/app/models/membership.rb
index 359c757..bac65e5 100644
--- a/app/models/membership.rb
+++ b/app/models/membership.rb
@@ -1,7 +1,8 @@
 class Membership < ActiveRecord::Base
   belongs_to :group
   belongs_to :user
 
   validates_presence_of :group
   validates_presence_of :user
+  validates_uniqueness_of :user_id, scope: :group_id
 end
diff --git a/test/factories/group.rb b/test/factories/group.rb
index cd92dc8..03a9cb4 100644
--- a/test/factories/group.rb
+++ b/test/factories/group.rb
@@ -1,5 +1,11 @@
 FactoryGirl.define do
   factory :group do
     sequence(:name) { |n| "group-#{n}" }
+
+    factory :group_with_users do
+      after(:create) do |group|
+        create_list(:user, 2).each { |u| group.users << u }
+      end
+    end
   end
 end
diff --git a/test/factories/user.rb b/test/factories/user.rb
new file mode 100644
index 0000000..d095dcd
--- /dev/null
+++ b/test/factories/user.rb
@@ -0,0 +1,6 @@
+FactoryGirl.define do
+  factory :user do
+    sequence(:email) { |n| "user-#{n}@example.com" }
+    password 'password'
+  end
+end
diff --git a/test/models/membership_test.rb b/test/models/membership_test.rb
new file mode 100644
index 0000000..f61ed09
--- /dev/null
+++ b/test/models/membership_test.rb
@@ -0,0 +1,13 @@
+require 'test_helper'
+
+class MembershipTest < ActiveSupport::TestCase
+
+  test 'single membership for a group' do
+    group = create(:group_with_users)
+    user = group.users.first
+
+    assert_raise ActiveRecord::RecordInvalid do
+      group.users << user
+    end
+  end
+end