More than one user can sign up for a meeting configured for a single person

In my calendar, several users can sign up for a meeting, but only one. If a meeting is already selected, another user should not be able to select it.

I have configured several meetings of 15, 30 minutes, and 1 hour in my calendar. I send the booking link to users so they can choose the time to reserve the meeting. The issue is that in some cases it works and in others it doesn't, allowing a second user to choose a meeting that is already booked. This forces me to cancel and assign another time. Can you help me with this?