count duplicate data in column

The first part of your problem: Values in A2:A12 enter in B2 and fill down:

Formula:

=IF(A2<>A1,MAX($B$1:B1)+1,B1)

	A	B
2	A	1
3	A	1
4	B	2
5	B	2
6	B	2
7	C	3
8	C	3
9	D	4
10	D	4
11	D	4
12	D	4