joshfeck · February 25, 2017 20:28 · Feb 25, 2017
diff --git a/ee_add_unique_email_validation.php b/ee_add_unique_email_validation.php
@@ -0,0 +1,24 @@
+<?php
+//* Please do NOT include the opening php tag, except of course if you're starting with a blank file
+
+function ee_add_unique_email_validation(){
+  wp_add_inline_script( 
+    'single_page_checkout',
+    'jQuery( document ).ready(function($) {
+      $(".ee-reg-qstn-email").addClass("unique");
+        $.validator.addMethod("unique", function(value, element) {
+        var parentForm = $(element).closest("form");
+        var timeRepeated = 0;
+        if (value != "") {
+          $(parentForm.find(":text")).each(function () {
+          if ($(this).val() === value) {
+              timeRepeated++;
+          }
+      });
+    }
+    return timeRepeated === 1 || timeRepeated === 0;
+    }, "* Duplicate! Please use a unique email address");
+    } );'
+  );
+}
+add_action( 'wp_enqueue_scripts', 'ee_add_unique_email_validation', 60 );