MVC - Coding selection of multiple times

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MVC4

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Hi!

So the problem that I'm having is to get a specific function for an appointment calendar.
Firstly, the scenario is that the system I need to build is for a business that has multiple technicians on hand. I need to develop an appointment calendar where the client can make an appointment by selecting it of a calendar. I have used DHTMLX to design the calendar and it works fine but the problem is when multiple users needs to make an appointment at the same time. The problem is the system won't allow me to create a new appointment without viewing the current appointment which is not suppose to happen.

For example: Client1 makes an appointment from 9am-10am, and then client2 makes an appointment from 9am-10am on the same day as well. This method is fine as there will be multiple technicians that are ready to be dispatched

I am currently using MVC in Visual Studio 2013

Any help given would be appreciated.
Thanks in advance