How not allow a new record to be added with the same Date and Employee ID as any previous records

I have a form which is a continuous form.  On the form is a date field and an EmployeeID combobox field.  The user may enter "9/28/2015" and "John Doe" for example. The bound field for the EmployeeID combo box is the PK number for that employee.   But if for example they have already entered a "9/28/2015" and "John Doe" and they try to enter another "9/28/2015" and "John Doe" , I want a message to appear letting them know they already entered "9/28/2015" and "John Doe" and not allow them to enter another one.

How can I do this?
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In the table design, create a unique index that includes the EmployeeID and the date.

Access will prevent a duplicate record from being saved regardless of how the save is initiated.  In your form, you might want to trap the error and make it more user friendly (use the On Error event) or you can proactively look for the duplicate values in the Beforeupdate event and cancel the event if you determine that the record already exists.  I'm not sure what error code will be raised so if you want to use the On Error event (my choice since it is most efficient), then add a message box and try to add a duplicate.  When the message box pops up, stop the code and print the error number in the debug window.  Then add code to trap for that specific error message and cancel the Access error message.
Private Sub Form_Error(DataErr As Integer, Response As Integer)
    Select Case DataErr
        Case ????
            Msgbox "This employee record already exists and cannot be added again.",vbokOnly
            Response = acDataErrContinue
        Case Else
            MsgBox "Please report this error to your technical support person.", vbOKOnly
            Response = acDataErrDisplay
    End Select
            End Sub

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