Instead of showing an error in the case where a user tries to reserve multiple times, simply perform a redirect.

[kavdev]

The error message is 'You have already reserved a slot. Please refresh this page.'. It might be better to just refresh the page for them.

[RAlmada]

I would tend to disagree. A page refreshing randomly causes confusion and may lead to people wondering why a different time than they clicked was chosen. The error message leaves it to the user to refresh while informing them of what happened.