select first occurrence of a table

Have a general SQL question:

I have a table that happens to have a some
duplicate entries in it (entire rows). I would like
to update one row and change one thing
about it, but it seems that I need a unique value
to do it; otherwise, I wind up changing multiple
rows.

For example: I have a table that looks like this

[snip table]

Table "survey_processed_dt"
Column | Type | Modifiers
-------------------+---------------------------+-----------
survid | character(8) |
processed_id | character varying(2) |
processed_dt | date |

[/snip table]

And the rows look like this:

[snip rows]

bcn=> select * from survey_processed_dt where survid = '02021000' and
processed_id = '05';
survid | processed_id | processed_dt
----------+--------------+--------------
02021000 | 05 | 2003-01-16
02021000 | 05 | 2003-01-16
(2 rows)

[/snip rows]

I want to change ONE of the rows ... is there a way to
just select the first occurrence of a row to update?

Suggestions?

Thanks!

-X