Restrict date and data entries

I have a subform "EmployeeWorkHis" where I want to prevent duplicate data
entry for certain "Option Groups" [Frame 40] and a singular date. As an
example on 4/9/2008 [OTdate] Record "1" I want to enter Option Group: 1, 3,
or 6 and then if Record "2" is required enter 4/9/2008 [OTdate] with Option
Group 2 or 5. I want to restrict to a singular date entry if Option Group 4
is selected. In other words if "Option Group" 4 is selected no other entry
may be entered for that date and if someone has entered record for that date
a warning message pops up stating " Record for this date must be deleted
before Option Group 4 and this date entry may be made"