auto- sequence number based on another column?

my problem:

column A contains a unique ID # for a well, in other columns are events for that well. Multiple events occur for each well (multiple rows with same unique ID #)

now I need to put into column B, a sequential number counting each event for each well.

I know I did not describe this well so let me try to show
A B
170570122000 1
170570122100 1
170570122100 2
170570125400 1
170570125400 2
170570125400 3
170570128400 1
170570128400 2


how to auto-generate column B?