Getting "duplicate key value violates unique constraint" on 2nd run of function.

Howdy all.

I've got a function that basically does this:

DELETE FROM test where id = $1
INSERT into test (id) values ($1);

id is the primay key, so it has to be unique.

First time I run it, works great.

If I run it again in the same session, I get
ERROR:  duplicate key value violates unique constraint "test_pkey"

If I log out and then log back in, it runs fine again (the first time).

Is there some setting for the function that I need to set to make this run correctly
every time?

Thanks

Dave