duplicate key violates unique constraint

Hi,

I'm trying to create a table with a PRIMARY KEY. The CREATE statement looks
like this:

CREATE TABLE  "projects" (
  "project_id" serial,
  "username" varchar(30) NOT NULL default '',
  "project_name" varchar(30) NOT NULL default '',
  PRIMARY KEY  ("project_id")
) ;

The problem is that sometimes, I would say 1 in 10 tries, when I use a INSERT
command I get the following error:

"duplicate key violates unique constraint"

The INSERT query is that:
"INSERT INTO projects (\"project_name\", \"username\") VALUES ('$project_name',
'$username')";

Can someone help me please?!!

Thanks,
Verônica