diff --git a/db/migrate/20250707080046_add_unique_constraint_to_user_assignments.rb b/db/migrate/20250707080046_add_unique_constraint_to_user_assignments.rb
new file mode 100644
index 000000000..90e615d96
--- /dev/null
+++ b/db/migrate/20250707080046_add_unique_constraint_to_user_assignments.rb
@@ -0,0 +1,24 @@
+# frozen_string_literal: true
+
+class AddUniqueConstraintToUserAssignments < ActiveRecord::Migration[7.2]
+  def up
+    # delete the duplicates
+    execute(
+      'WITH uniq AS
+      (SELECT DISTINCT ON (user_id, assignable_type, assignable_id, team_id) * FROM user_assignments)
+      DELETE FROM user_assignments WHERE user_assignments.id NOT IN
+        (SELECT id FROM uniq)'
+    )
+
+    remove_index :user_assignments, name: 'index_user_assignments_on_assignable'
+    add_index :user_assignments,
+              %i(assignable_type assignable_id user_id team_id),
+              unique: true,
+              name: 'index_user_assignments_on_unique_assignable_in_team'
+  end
+
+  def down
+    add_index :user_assignments, %i(assignable_type assignable_id), name: 'index_user_assignments_on_assignable'
+    remove_index :user_assignments, name: 'index_user_assignments_on_unique_assignable_in_team'
+  end
+end