Do I have to lock table in this case?

I have a table like this:
 id primary key
 unique

I need to do:
 - if 'unique' exists in table, return appropriate id
else insert into table new id.
Now doing:
select id where unique=%s 
insert/update 
creates a race condition. The application can theoretically rollback &
redo the whole transaction (not simple in some cases). What is the
preferred solution to this problem (I got about 3 such cases in one simple
application).

Ondrej

--
It is better to remain childless than to father an orphan.