Excel Dynamic Array - Resetting a Row Count

mswartzentruber 

Two ways to do it depending on your setup.

with a table:

=LET(k, COUNTIFS($B$2:B2, B2, $C$2:C2, C2), B2 & "-" & C2 & "-" & k)

with dynamic array:

=LET(
    i, SEQUENCE(ROWS(customer)),
    counter, LAMBDA(i, each_customer, each_item,
        LET(
            k, COUNTIFS(TAKE(customer, i), each_customer, TAKE(item, i), each_item),
            each_customer & "-" & each_item & "-" & k
        )
    ),
    MAP(i, customer, item, counter)
)

The second option is more exciting but the first option may be more sensible if the customer data is being used as data entry primarily.