Creating users with duplicate email is not erroring

I am currently using alog in another project where two tables are being updated at the same time.

The `venue` table has a `many to many` association with the `users` table. When a user of the site creates a new `venue` they can also create a new `user` at the same time.

However, for some reason the user changeset function is not working as expected here. It is allowing multiple users with the same email address to be created.

This does not happen when you try to create a user on their own (e.g. Call the `User.insert` function directly with a `user changeset`)