Problem in events overlapping

Good Morning
I have a big problem in the management of event overlapping.
Even if i turn on the option to prevent event overlapping, it’s possible to make two events in the same moments.

After many attempt I figure out how.

I use joomla 1.5.14 and scheduler 3.0.

If one user open the pop-up for take an appointment for example at, than an other user open the pop-up for take an appointment at and the first user have not finish yet with his prenotation, both of them, when they confirm, create the event at the same hour.

I need a big help, i’m in trouble.

At least I can even pay for a solution.
help me plz.

Should be implemented a check on db for see if the hour is free when you click to confirm the appointment.
But now i can’t figure how, i have to study the problem.