What is the postgres version of mysql's "ON DUPLICATE KEY"

I have a table with columns
(product_id,related_product_id,related_counter)

If product A is related to product B then a record should be created,
if the record already exists then the related_counter should be
incremented.

This is very easy to do with MySQL using INSERT... ON DUPLICATE KEY.
Standard or not, it is very usefull.

Is there a way to catch the insert error. For example...

INSERT INTO related_products (product_id,related_product_id) VALUES
(?,?);
IF (???error: duplicate key???) THEN
UPDATE related_products SET related_counter = related_counter + 1;
END IF;

-Nick