diff --git a/app/models/registration.rb b/app/models/registration.rb
index 3ede8051b..6f6d2d310 100644
--- a/app/models/registration.rb
+++ b/app/models/registration.rb
@@ -24,6 +24,21 @@ class Registration < ApplicationRecord
   delegate :course, to: :exam
   delegate :term, to: :course
 
+  validates :user, uniqueness: { scope: :exam_version }
+
+  # note: we are not sure whether the current reg will show up in the association,
+  # so this validation remains separate from the previous uniqueness check
+  validate :user_exam_uniqueness
+  def user_exam_uniqueness
+    other_reg_exists =
+      user
+      .registrations
+      .where(exam_version: exam.exam_versions)
+      .where.not(exam_version: exam_version)
+      .any?
+    errors.add(:user, 'already has a registration for another version of that exam') if other_reg_exists
+  end
+
   def room_version_same_exam
     return unless room
 
diff --git a/test/models/user_test.rb b/test/models/user_test.rb
index 83cbe5453..8421daa6e 100644
--- a/test/models/user_test.rb
+++ b/test/models/user_test.rb
@@ -10,4 +10,21 @@ class UserTest < ActiveSupport::TestCase
   test 'admin factory builds admins' do
     assert build(:admin).admin?
   end
+
+  test 'cannot create two registrations for the same ExamVersion-User pair' do
+    r1 = create(:registration)
+    r2 = build(:registration, user: r1.user, exam_version: r1.exam_version)
+    assert_not r2.valid?
+    assert_match(/has already been taken/, r2.errors.full_messages.to_sentence)
+    assert_not r2.save
+  end
+
+  test 'cannot create two registrations for the same Exam-User pair' do
+    r1 = create(:registration)
+    second_version = create(:exam_version, exam: r1.exam)
+    r2 = build(:registration, user: r1.user, exam_version: second_version)
+    assert_not r2.valid?
+    assert_match(/has a registration for another version/, r2.errors.full_messages.to_sentence)
+    assert_not r2.save
+  end
 end