VBA for Unique Pick Numbers for unique Order Numbers

I am trying to write VBA code for a macro that will assign a unique pick number for each unique order number. Matching order numbers will get matching pick numbers. In other words, orders: A, A, A, B, & B will be assigned pick numbers 1,1,1,2,& 2. How do I go about doing so? I did write basic pseudo code but I am unsure its accuracy to what I want to accomplish:

For each ROW:

if PO in ROW-1 == PO in ROW-1 then:
PN=PN
ELSE:
PN=PN+1

NEXT ROW

For visual example, I provided a link to a screenshot taken from EXCEL 2010.

SCREENSHOT

Hi you don't need a macro.

Just put 9000 in A1 and in A2 copied down

Formula:

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