diff --git a/app/models/mentor_at_school_period.rb b/app/models/mentor_at_school_period.rb
index 22a9b446..b10eb70a 100644
--- a/app/models/mentor_at_school_period.rb
+++ b/app/models/mentor_at_school_period.rb
@@ -13,6 +13,10 @@ class MentorAtSchoolPeriod < ApplicationRecord
             notify_email: true,
             allow_nil: true
 
+  validates :email,
+            email_uniqueness: true,
+            allow_nil: true
+
   validates :started_on,
             presence: true