[DefaultValue(AppointmentConflictsMode.Allowed)]
public AppointmentConflictsMode AllowAppointmentConflicts { get; set; }

<DefaultValue(AppointmentConflictsMode.Allowed)>
Public Property AllowAppointmentConflicts As AppointmentConflictsMode

public Form1() {
    InitializeComponent();
    SchedulerResLocalizer.Active = new CustomSchedulerResLocalizer();
}

public class CustomSchedulerResLocalizer : SchedulerResLocalizer {
    public override string GetLocalizedString(SchedulerStringId id) {
        if(id == SchedulerStringId.Msg_Conflict)
            return "_YOUR_CUSTOM_MESSAGE_TEXT_";
        else 
            return base.GetLocalizedString(id);
    }    
}

Public Sub New()
    InitializeComponent()
    SchedulerResLocalizer.Active = New CustomSchedulerResLocalizer()
 End Sub

Public Class CustomSchedulerResLocalizer
    Inherits SchedulerResLocalizer

    Public Overrides Function GetLocalizedString(ByVal id As SchedulerStringId) As String
        If id Is SchedulerStringId.Msg_Conflict Then
            Return "_YOUR_CUSTOM_MESSAGE_TEXT_"
        Else
            Return MyBase.GetLocalizedString(id)
        End If
    End Function
End Class

schedulerControl1.OptionsCustomization.AllowAppointmentConflicts = AppointmentConflictsMode.Custom;
//...
private void SchedulerControl1_AllowAppointmentConflicts(object sender, AppointmentConflictEventArgs e) {
    e.Conflicts.Clear();
    FillConflictedAppointmentsCollection(e.Conflicts, e.Interval, ((SchedulerControl)sender).DataStorage.Appointments.Items, e.AppointmentClone);
}

void FillConflictedAppointmentsCollection(AppointmentBaseCollection conflicts, TimeInterval interval,
    AppointmentBaseCollection collection, Appointment currApt) {
    for(int i = 0; i < collection.Count; i++) {
        Appointment apt = collection[i];
        if(new TimeInterval(apt.Start, apt.End).IntersectsWith(interval) & !(apt.Start == interval.End || apt.End == interval.Start)) {
            if(apt.ResourceId == currApt.ResourceId) {
                conflicts.Add(apt);
            }
        }
        if(apt.Type == AppointmentType.Pattern) {
            FillConflictedAppointmentsCollection(conflicts, interval, apt.GetExceptions(), currApt);
        }
    }
}

schedulerControl1.OptionsCustomization.AllowAppointmentConflicts = AppointmentConflictsMode.Custom
'...
Private Sub SchedulerControl1_AllowAppointmentConflicts(ByVal sender As Object, ByVal e As AppointmentConflictEventArgs)
    e.Conflicts.Clear()
    FillConflictedAppointmentsCollection(e.Conflicts, e.Interval, DirectCast(sender, SchedulerControl).DataStorage.Appointments.Items, e.AppointmentClone)
End Sub

Private Sub FillConflictedAppointmentsCollection(ByVal conflicts As AppointmentBaseCollection, ByVal interval As TimeInterval, ByVal collection As AppointmentBaseCollection, ByVal currApt As Appointment)
    For i As Integer = 0 To collection.Count - 1
        Dim apt As Appointment = collection(i)
        If (New TimeInterval(apt.Start, apt.End)).IntersectsWith(interval) And Not (apt.Start = interval.End OrElse apt.End = interval.Start) Then
            If apt.ResourceId = currApt.ResourceId Then
                conflicts.Add(apt)
            End If
        End If
        If apt.Type = AppointmentType.Pattern Then
            FillConflictedAppointmentsCollection(conflicts, interval, apt.GetExceptions(), currApt)
        End If
    Next i
End Sub

this.schedulerControl1.OptionsCustomization.AllowAppointmentConflicts = AppointmentConflictsMode.Custom;
this.schedulerControl1.AllowAppointmentConflicts+=new AppointmentConflictEventHandler(schedulerControl1_AllowAppointmentConflicts);

schedulerControl1.OptionsCustomization.AllowAppointmentConflicts = AppointmentConflictsMode.Custom;
schedulerControl1.AllowAppointmentConflicts += SchedulerControl1_AllowAppointmentConflicts;

AddHandler appointmentsTableAdapter.Adapter.RowUpdated, New SqlRowUpdatedEventHandler(AddressOf appointmentsTableAdapter_RowUpdated)
schedulerControl1.OptionsCustomization.AllowAppointmentConflicts = AppointmentConflictsMode.Custom
AddHandler schedulerControl1.AllowAppointmentConflicts, New AppointmentConflictEventHandler(AddressOf schedulerControl1_AllowAppointmentConflicts)

schedulerControl1.Start = Date.Now
schedulerControl1.OptionsCustomization.AllowAppointmentConflicts = AppointmentConflictsMode.Custom
AddHandler schedulerControl1.AllowAppointmentConflicts, AddressOf SchedulerControl1_AllowAppointmentConflicts