How to prevent duplicate data entry in a text box??

Hi I have a simple form for data entry, the MDB file contains 4 fields, and I have a form with 4 text boxes bounded to the MDB file. My question is how do I prevent a duplicate entry on the text box??

I have a simple procedure but it gives me runtime error 3426 ??

' After entering data hit the check_duplicate button to
' check for duplicate entry
Private Sub CheckDuplicateButton_Click()
    strBookmark = ContractData.Recordset.Bookmark
    ' CQuote is the textbox name which bound to the MDB file field [Quote #]
    strCriterion = "[Quote #]= '" & UCase(CQuote) & "'"

    ContractData.Recordset.FindFirst strCriterion
    If ContractData.Recordset.NoMatch Then
        ContractData.Recordset.Bookmark = strBookmark
    Else
        MsgBox ("Duplicate Contract # Entry!!")
    End If
End Sub

If I enter a quote# which previously exist it gives me runtime error 3426??

Any idea from you guys or any better way?? Oh I am using VB4.0.


Thanks