I try to replicate my email server db to another server for backup. When there is an update on the master db, sometimes it will miss data inserting into one of the tables on the slave server. It will add the row but the data inside are all "Null". Then, it will try to add that row again and it will generate the duplicate primary key error which causes the replication to stop. On the internet, a lot of people say to skip the error, but I don't think it is the right way to do it because it will create the inconsistency on the slave db.