Prevent Duplicate Record in Database

I have a table with 4 different time slot, and user can choose to make an appointment everyday. I am trying to write a code to prevent same timeslot booking. However, when i tried to book appointment at any of the date. The result is "Not available". protected void Booking(object sender, EventArgs e) { bool UserExist; UserExist = this.UserExist(); if (UserExist == true) { string strConnectionString = ConfigurationManager.ConnectionStrings["ConnectionString"].ConnectionString; //create connection SqlConnection myConnect = new SqlConnection(strConnectionString); string strCommandText = "Insert Into CaregiverBooking (CaregiverID, UserName, PatientName, PatientAddress, SpecialRequest, TodayDate, Time )"; strCommandText += " Values (@Careg ...