I am testing this widget for my tennis club. And i can easily double book timeslots. I am a dev myself, so if you’d like more info, just ask.
Also, there is no way to cancel a booking as enduser or admin. Which makes it quite unusable.
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Hey there and welcome aboard, @Martijn_Michel 
You are right, unfortunately, there is no way to cancel appointments now. However, we have this idea on the Wishlist and if it gets more votes, we’ll try to consider it. You can upvote it here - End user can change or cancel a booking
As for the double book time slots, you mean that the slots don’t get blocked after reservation, right?
If this is the case, you should connect your widget to Google Calendar and everything will work fine:
However, if I misunderstood you, please describe the issue in more detail 