I think I finally found the problem. You are doing your test on your original workbook, aren't you?
On the workbook I attached, You'll see that I added 2 titles in row 1. Column G = ??? and Column H = DUP
When you run my macro on your original workbook, the Last_Col variable is set to 6 as you stated in one of your post. Column 6 being F which contains e-mail addresses.
With my modification of the titles, the Last_Col variable has the value of 8 which is column H where we want to put the duplicate number.
Column G contains alpha data whcih causes the error. Column H contains intergers and should not cause any error.
Either upload my workbook or do the modification to your title row (row 1) and every thing should work just fine.
Sorry about this issue and all the time I took to realize it.
If you want the numbering to start at number 1, then change this line of code
to
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