=A2&"."&(COUNTIF(INDIRECT("A$1:A"&ROW()-1), A2)+1)

The SupplierCode needs to be in column A, this would go in B2. Which would result in this:

[img]https://photos-3.dropbox.com/t/0/AAAlUyo...-VVelKcthEp3clQ[/img]

What the formula does is look back up the A column and counts all the supplier codes that match the supplier code on the current row. Then it adds one to the count and uses that for the sequential number. So each supplier gets their own incrementing set of numbers.

I am probably being over complicated with the INDIRECT, this works just as well:

=A2&"."&(COUNTIF(A$1:A1, A2)+1)