Prevent duplicate entries in a ms access table

I am new to ms access, I have prepared a database for a school where I want to apply duplicate entry restrictions.

I want to stop duplicate entry in same month. Type of studentID is number while the month is linked with another table where its type is text. I have tried many options but type mismatch error occurs. The data is being entered from a form and I need to apply validation at the time of form update

Private Sub Form_BeforeUpdate(Cancel As Integer)
Dim x As Integer

x = DCount("StudentID", "tblFeeVoucherGenerate", "StudentID =" & Me.StudentID & " ")
If x > 0 Then
 MsgBox "Fee Already Recorded for this month"
 Me.Undo
 Exit Sub
End If
DoCmd.RunCommand acCmdSaveRecord
MsgBox "Record has been Saved"
DoCmd.RunCommand acCmdRecordsGoToNew

End Sub

I want to add month condition to record a student fee once in a month.

Text field parameters need apostrophe delimiters (date/time uses # character). Consider:

x = DCount("StudentID", "tblFeeVoucherGenerate", "StudentID =" & Me.StudentID & _
     " AND MonthFieldName='" & Me.tbxMonth & "'")

Month is a reserved word - advise not to use it as field name.

DCount function does not cross tables and can be slow if table is big. Consider making a query to join the tables named qryFeeVoucherGenerate

SELECT tblFreeVoucherGenerated.StudentID AS StudentID, 
tblWithNameOfDateField.NameOfDateFiled AS NameOfDateField FROM tblFreeVoucherGenerated
LEFT JOIN
tblWithNameOfDateField 
ON tblFreeVoucherGenerated.StudentID = tblWithNameOfDateField.StudentID
WHERE DATEDIFF( 'm' , tblWithNameOfDateField.DonationDate, Date()) = 0;

Save the above query with name qryFeeVoucherGenerated then use:

 Private Sub StudentID_AfterUpdate()

'If you decide to create a textbox for the DateField the use:
'DCount("*", "[qryFeeVoucherGenerate]", "[StudentID] = '" & Me.StudentID.Value & "' AND CDATE([NameOfDateField]) = '" & Me.NameOfMonthFiled.Value & "'") > 0

 If DCount("[StudentID]", "[qryFeeVoucherGenerate]", "[StudentID] = '" & Me.StudentID.Value & "'") > 0 Then

 MsgBox "Fee Already Recorded for this month"
 Me.Undo
 Exit Sub
 End If

 MsgBox "Record has been Saved"
 DoCmd.RunCommand acCmdRecordsGoToNew

 End Sub