Data Validation - URGENT PLEASE HELP

Could anyone please let me know urgently how you can set data validation for a combo box please. I want to stop the user entering duplicate enteries from the combo box. Or if there is any VBA code that anyone knows to do this that would be great.

Cheers for you help.

Elodie

The VBA code used to set up my combo box is:
Sub AddDropDown(Target As Range)

Dim ddBox As DropDown
Dim vaProducts As Variant
Dim i As Integer

vaProducts = Array(Sheet2.Cells.Range("A1"), Sheet2.Cells.Range("B1"), Sheet2.Cells.Range("C1"), Sheet2.Cells.Range("D1"), Sheet2.Cells.Range("E1"), Sheet2.Cells.Range("F1"), Sheet2.Cells.Range("G1"), Sheet2.Cells.Range("H1"))
With Target
Set ddBox = Sheet3.DropDowns.Add(.Left, .Top, .Width, .Height)
End With
With ddBox
.OnAction = "EnterProdInfo"
For i = LBound(vaProducts) To UBound(vaProducts)
.AddItem vaProducts(i)
Next i
End With
End Sub

Private Sub EnterProdInfo()
Dim vaPrices As Variant

vaPrices = Array("A1", "B1", "C1", "D1", "E1", "F1", "G1", "H1")
With Sheet3.DropDowns(Application.Caller)
.TopLeftCell.Value = .List(.ListIndex)
.TopLeftCell.Offset(0, 2).Value = vaPrices(.ListIndex - Array(0, 1)(1))
.Delete
End With