autoincrement formula

[johnydvacka]

Hi Folks,
please, I would like to ask you for formula:
If the number in column A is equal to the previous record (in the same column), add 1 to value in the column B.
If the number in column A is different (first), start numbering in column B from number 01. For example:
0112 | 01
0112 | 02
0113 | 01
0114 | 01
0114 | 02
0114 | 03 ...
Thank you.