Count How many times a value appeared in a column(excel)

Question

Ok, So I have an excel file that have a column that may contain duplicate value like this

PO NUMBER        PO COUNT
P100

Answer 1

Assuming the data as you give it is in A1:B18 (with headers in row 1), enter this in B2:

=IF(A1<>A2,MATCH(TRUE,INDEX(A2:A$1000<>A2,),)-1,"")

Copy down as required.

Amend the 1000 to a sufficiently higher row reference if necessary.