check text box after update for duplicate

[smahdih]

i have a form with many fields



when i type a duplicate value for primary key and save record duplicate error(attach it) is shown.

i want after update primary key check for duplicate, if is not duplicate other field allowed fill

how can do this?

[June7]

Use the BeforeUpdate event to check for existing value.

If Not IsNull(DLookup("fieldname","tablename","fieldname= " & Me.textbox)) Then
Cancel = True
MsgBox "ID already used."
End If