Update with a Repeating Sequence

I've got a table with repeated records that I want to make unique by
adding a sequence code of 0,1,2,...,n for each set of repeated records.
Basically, I want to turn:
 field_id | seq
----------+-----
        1 |   0
        2 |   0
        3 |   0
        3 |   0
        3 |   0
        4 |   0
        4 |   0
        5 |   0
        6 |   0
into:
 field_id | seq
----------+-----
        1 |   0
        2 |   0
        3 |   0
        3 |   1
        3 |   2
        4 |   0
        4 |   1
        5 |   0
        6 |   0

What's the best way to that?

TIA,
- Bill Thoen