From my understanding INSERT IGNORE inserts a new entry if it doesn't already exists, if it does, ignore it. So I've been trying to do that for a while and it doesn't seem to be working. Here's my attempt:
insert insert ignore into rss_feeds (md5sum) values ("1234");
Query OK, 1 row affected (0.00 sec)
mysql> insert ignore into rss_feeds (md5sum) values ("1234");
Query OK, 1 row affected (0.00 sec)
mysql> insert ignore into rss_feeds (md5sum) values ("1234");
Query OK, 1 row affected (0.00 sec)
mysql> insert ignore into rss_feeds (md5sum) values ("1234");
Query OK, 1 row affected (0.00 sec)
mysql> select * from rss_feeds where md5sum="1234";
+------+--------+----------+---------+----------+--------+--------+---------+
| link | source | headline | updated | inserted | md5sum | itemid | emailed |
+------+--------+----------+---------+----------+--------+--------+---------+
| NULL | NULL | NULL | NULL | NULL | 1234 | NULL | NULL |
| NULL | NULL | NULL | NULL | NULL | 1234 | NULL | NULL |
| NULL | NULL | NULL | NULL | NULL | 1234 | NULL | NULL |
| NULL | NULL | NULL | NULL | NULL | 1234 | NULL | NULL |
+------+--------+----------+---------+----------+--------+--------+---------+
4 rows in set (0.00 sec)