diff --git a/src/main/java/net/frozenorb/apiv3/route/users/POSTUsersIdRegister.java b/src/main/java/net/frozenorb/apiv3/route/users/POSTUsersIdRegister.java
index 7a6cfcb..54aff97 100644
--- a/src/main/java/net/frozenorb/apiv3/route/users/POSTUsersIdRegister.java
+++ b/src/main/java/net/frozenorb/apiv3/route/users/POSTUsersIdRegister.java
@@ -53,6 +53,14 @@ public final class POSTUsersIdRegister implements Handler<RoutingContext> {
             return;
         }
 
+        BlockingCallback<User> sameEmailCallback = new BlockingCallback<>();
+        User.findByEmail(email, sameEmailCallback);
+
+        if (sameEmailCallback.get() != null) {
+            ErrorUtils.respondInvalidInput(ctx, email + " is already in use.");
+            return;
+        }
+
         user.startRegistration(email);
         BlockingCallback<UpdateResult> callback = new BlockingCallback<>();
         user.save(callback);