diff --git a/app/models/user_assignment.rb b/app/models/user_assignment.rb
index fda354e47..4f21d82e5 100644
--- a/app/models/user_assignment.rb
+++ b/app/models/user_assignment.rb
@@ -7,4 +7,6 @@ class UserAssignment < ApplicationRecord
   belongs_to :assigned_by, class_name: 'User', optional: true
 
   enum assigned: { automatically: 0, manually: 1 }, _suffix: true
+
+  validates :user, uniqueness: { scope: :assignable }
 end