is there a select for update insert if not exist type command?

I have a project where I will have two clients essentially doing the
same things at the same time. The idea is that if one has already done the
work, then the second one does not need to do it.

I was hoping that adding a task related unique identifier to a table
could be used to coordinate these client, something like a primary key and using
select for update.

The challenge I have is during the initial insert. One of the two clients will cause postgresql
to log an error, which I would rather avoid (just seems dirty).

Here is the time line;

Both clients A and B becomes aware to do a task

Client A or client B issues the "select for update ... if not exist do insert" type command
The other client gets blocked on the "select for update.

First client finishes insert/updates to record that it has delt with the task

second client gets unblocked and reads the record realizing that the first client delt with the task already.

It is the "select for update ... if not exist do insert" type command that I am ignorant of how to code.

Anyone care to school me?

Evan.