This would be in the BeforeUpdate event - after the update it is too late
Code:
Dim CheckDupe
CheckDupe=DlookUp("pk_field","Beplan_rooster","...criteria, such as: Heading='" & Me!Heading & "' AND Date.....")
If Not Isnull(CheckDupe) Then
error message
Cancel=True
End If
Do not use "Date", it is a reserved word in Access and will cause you problems.
To figure out the syntax of the criteria for the DLookUp, create a new query and add some made-up criteria. Then go to the SQL and copy from WHERE, the syntax will match. Put single quotes around text fields and "#" around date fields.