如 INSERT语法:

没有IGNORE的情况下，与表中现有的UNIQUE索引或PRIMARY KEY值重复的行会导致重复键错误，并且该语句将中止.使用IGNORE，仍不会插入该行，但不会发出错误.

without IGNORE, a row that duplicates an existing UNIQUE index or PRIMARY KEY value in the table causes a duplicate-key error and the statement is aborted. With IGNORE, the row still is not inserted, but no error is issued.

您需要在md5sum上定义UNIQUE索引:

ALTER TABLE rss_feeds ADD UNIQUE INDEX (md5sum);