diff --git a/langs/en/signup.json b/langs/en/signup.json
index dfa8b50..a0cc189 100644
--- a/langs/en/signup.json
+++ b/langs/en/signup.json
@@ -10,5 +10,6 @@
     "Please enter your username (4-100 characters, alphanumeric).": "Please enter your username (4-100 characters, alphanumeric).",
     "That password is one of the most popular and insecure ever, make a better one.": "That password is one of the most popular and insecure ever, make a better one.",
     "Account creation not allowed.  Contact the site administrator for an account.": "Account creation not allowed.  Contact the site administrator for an account.",
-    "CAPTCHA answer incorrect.": "CAPTCHA answer incorrect."
+    "CAPTCHA answer incorrect.": "CAPTCHA answer incorrect.",
+    "That email address is already in use.": "That email address is already in use."
 }
diff --git a/signup/index.php b/signup/index.php
index 5ced7a1..c24a9a9 100644
--- a/signup/index.php
+++ b/signup/index.php
@@ -161,6 +161,9 @@ if ($passrank !== FALSE) {
 if (!empty($_POST['email']) && !filter_var($_POST['email'], FILTER_VALIDATE_EMAIL)) {
     showHTML($Strings->get("That email address doesn't look right.", false));
 }
+if (!empty($_POST['email']) && $database->has("accounts", ["email" => strtolower($_POST['email'])])) {
+    showHTML($Strings->get("That email address is already in use.", false));
+}
 if (empty($_POST['name'])) {
     showHTML($Strings->get("Enter your name.", false));
 }
@@ -179,7 +182,7 @@ if ($_POST["textcaptcha"] != "DISABLE" . hash("sha1", hash("md5", date("Ymd"))))
 
 // Create account
 
-$userid = User::add($_POST['username'], $_POST['password'], $_POST['name'], (filter_var($_POST['email'], FILTER_VALIDATE_EMAIL) ? $_POST['email'] : null));
+$userid = User::add($_POST['username'], $_POST['password'], $_POST['name'], (filter_var($_POST['email'], FILTER_VALIDATE_EMAIL) ? strtolower($_POST['email']) : null));
 $signinstr = $Strings->get("sign in", false);
 $redirect = urlencode($_POST["redirect"]);
 $code = urlencode($_POST["code"]);