how to increment column B based on column A value?

Hi -
I have column A with values that repeat and then change ever few rows.
I would like to create a formula for column B to increment once for each occurrence of the A value.

Starting with
'A'
10
10
20
20
20

I would like to create these values in column B
'A' 'B'
10 1
10 2
20 1
20 2
20 3

So finally (I could do this separately) to create column 'C' as a concatenation of 'A' and 'B'
'A' 'B' 'C'
10 1 101
10 2 102
20 1 201
20 2 202
20 3 203

Thank you in advance.

at B1
=COUNTIF(A$1:A1,A1)
at C1
=A1& B1
or direct
A1&COUNTIF(A$1:A1,A1)

This worked perfectly. Thank you!