Allow registered users to book without the need of logging in

When a user books an event for the first time his account gets created.

When a user returns to the website and is not logged-in he will not be able to book another event without the need to login first. He will get the message that his email address is already registered and that he needs to log in first.
In my view this is not very user-friendly.

How do I achieve that a returning user can book another event with his already registered email address WITHOUT the need for him to log in first?