#include <db.h>  DB->set_dup_compare(DB *db,     int
(*dup_compare_fcn)(DB *db, const DBT *dbt1, const DBT *dbt2));
Set the duplicate data item comparison function. The comparison
function is called whenever it is necessary to compare a data
item specified by the application with a data item currently
stored in the database. Calling DB->set_dup_compare() implies
calling DB->set_flags() with the DB_DUPSORT flag.  If no
comparison function is specified, the data items are compared
lexically, with shorter data items collating before longer data
items.  The DB->set_dup_compare() method may not be called after
the DB->open() method is called. If the database already exists
when DB->open() is called, the information specified to
DB->set_dup_compare() must be the same as that historically used
to create the database or corruption can occur.  The
DB->set_dup_compare() method returns a non-zero error value on
failure and 0 on success.    The dup_compare_fcn function is the
application-specified duplicate data item