This should do it even if you have more than 2 dups. Must be SORTED

option explicit
Sub uniquenums()
Dim i As Long
Dim un As Long
Dim mc As Double
un = 1
For i = 2 To Cells(Rows.Count, 1).End(xlUp).Row
mc = Application.CountIf(Columns(1), Cells(i, 1))
If mc 1 And Cells(i - 1, 1) < Cells(i, 1) Then
Cells(i, 2).Resize(mc) = un
un = un + 1
End If
Next i
Columns(2).NumberFormat = "0000"
End Sub

--
Don Guillett
Microsoft MVP Excel
SalesAid Software

"Heather" wrote in message
...
Hi All,
I need to assign a unique number to a set of duplicates all in one column
in
Excel 2007.

so columnA will has about 9000 numbers, some of them unique, and others
are
duplicates of 2-4 approx.

I used to conditional formatting to show which are duplicates, but need to
be able to assign a unique number to each set duplicates, that will be in
sequential order...

e.g.
ColumnA ColumnB(unique ID)
01233 0001
01233 0001
01234 -
01255 0002
01255 0002
etc....

Any ideas please? I don't know how to do programming, just formulas in
excel.
Thanks
Heather