Inserting from multiple processes?

I'm using Postgres 8.4 on RHEL 6 and I need to support inserting data into a table from multiple processes with there occasionally being duplicates from the different processes. Here's a simple example table:
CREATE TABLE test (tutc TIMESTAMP WITHOUT TIME ZONE, id INTEGER, value INTEGER, PRIMARY KEY (tutc, id));
If I do the following query from 2 processes, then it's fine:
INSERT INTO test (tutc, id, value) SELECT '2015-01-01 01:02:03', 4, 5 WHERE NOT EXISTS (SELECT 1 FROM test WHERE tutc='2015-01-01 01:02:03' AND id=4);

Using an "upsert" type of function, like the one shown in the documentation ( see http://www.postgresql.org/docs/8.4/static/plpgsql-control-structures.html#PLPGSQL-UPSERT-EXAMPLE ) seems like it might work, but I don't need to support updating and was hoping to not have to use a custom function. So is there some way to catch the unique_violation exception without creating a function? Or some other solution to this?

Thanks,

Dave