Removing duplicate rows in table

I've no idea how I entered multiple, identical rows in a table but I want to
delete all but one of these rows.

Here's an example:

bustrac=# select * from projects where proj_nbr = '4242.01';
  proj_nbr |   proj_name    | start_date |  end_date  |  description  | notes 
----------+----------------+------------+------------+---------------+-------
  4242.01  | Expert witness | 2008-10-15 | 2008-10-28 | Consol Energy |
  4242.01  | Expert witness | 2008-10-15 | 2008-10-28 | Consol Energy |
  4242.01  | Expert witness | 2008-10-15 | 2008-10-28 | Consol Energy |
  4242.01  | Expert witness | 2008-10-15 | 2008-10-28 | Consol Energy | 
(4 rows)

How do I clean this up so there's only a single row for this project number?

TIA,

Rich