Steffan, the fix for issue1876683 (that you suggested) seems to have
been rolled back or missed for mysql.

The upshot is that a unique index on the __retired__ and key value is 
missing for the mysql backend.

Do you have any idea what would happen if I were to make the change 
specified in this ticket to back_mysql? Would it crash people's
databases? Do you remember how you recovered from the duplicate
entries with the same key in your database?

Thanks.