How to Prevent Duplicate entries in excel vba.

Basic Concept

Dim dsh As Worksheet

Set dsh = Sheets("Dealer Boxes")

Dim m As Long

For m = 2 To dsh.Range("D100000").End(xlUp).Row

If Application.WorksheetFunction.CountIf(dsh.Range("D" & 2, "D" & m), Me.TextBox1.Value) Then

MsgBox "Already distributed, Please try another !"

Exit Sub

End If

Next m