Check if data is already exist before adding a new data

Hello! So, I'm currently having a problem in my data entry. Just want to ask if how can I avoid duplicates? So on my worksheet, I have an ADD sheet that the user is going to use to enter the data. Then I have the DATA sheet where all of the data is. The unique key is the STUDENT ID. Thank you in advance!

Code:

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Sub add_stud()

    If Application.CountIf(Worksheets("Data").Columns(3), Range("a_studID").Value) > 0 Then
        MsgBox "Existing student ID in database! Quitting...", vbOKOnly, "Duplicate entry!"
        Exit Sub
    End If
   
    With Worksheets("Data")
        next_row = .Range("C" & .Rows.Count).End(xlUp).Offset(1).Row
        .Cells(next_row, 3).Value = Range("a_studID").Value
        .Cells(next_row, 4).Value = Range("a_lname").Value
        .Cells(next_row, 5).Value = Range("a_fname").Value
        .Cells(next_row, 6).Value = Range("a_ext").Value
        .Cells(next_row, 7).Value = Range("a_mname").Value
    End With
   
    ThisWorkbook.Save
   
    MsgBox "Data has been saved!", vbOKOnly + vbInformation, "SF10"
   
End Sub

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Quote:

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Originally Posted by mancubus

Code:

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Sub add_stud()

    If Application.CountIf(Worksheets("Data").Columns(3), Range("a_studID").Value) > 0 Then
        MsgBox "Existing student ID in database! Quitting...", vbOKOnly, "Duplicate entry!"
        Exit Sub
    End If
   
    With Worksheets("Data")
        next_row = .Range("C" & .Rows.Count).End(xlUp).Offset(1).Row
        .Cells(next_row, 3).Value = Range("a_studID").Value
        .Cells(next_row, 4).Value = Range("a_lname").Value
        .Cells(next_row, 5).Value = Range("a_fname").Value
        .Cells(next_row, 6).Value = Range("a_ext").Value
        .Cells(next_row, 7).Value = Range("a_mname").Value
    End With
   
    ThisWorkbook.Save
   
    MsgBox "Data has been saved!", vbOKOnly + vbInformation, "SF10"
   
End Sub

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It worked! Thank you so much!

you are welcome.