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...
I have a list of values that are all the same length and end take the
form xxxxxxLO
where x is any letter A-Z.

Several rows contain the same value and I want to make these unique
according to the following rule:

locate next value
check if it's a duplicate
if not move on
else ignore the first instance and replace LO with AB, AC, AD...etc
for all subsequent duplicates

Does this do what you want?

Public Sub renameUpTo25duplicates()
Dim x As String, y As String
Dim i As Integer, j As Integer

i = 2
j = 66 'B
With Sheet1
x = .Range("A" & (i - 1))
y = .Range("A" & i)
Do Until y = ""
If x = y Then
.Range("A" & i) = Left(y, 6) & "A" & Chr(j)
j = j + 1 'Note: chr(91) = "["
Else
j = 66 'B
x = y
End If
i = i + 1
y = .Range("A" & i)
Loop
End With
End Sub

--
roger