When using the onduplicatekey_update option in AR-extentions composite keys that can contain NULL values will not work.
For example say you were to import a list of people. Your table used First Name, Middle Name and Last Name as your composite key. The entry “Bob Dole” would not match the person in the database with the First Name Bob, Middle Name NULL and Last Name Dole. This entry would be inserted again and your table will contain duplicates.
To work around this either change your composite key to exclude the NULL values or enter the values in as empty strings or something other than NULL.
