When importing data from CSV the server performs asearch by rec_name to determine the target record for Many2One. From the error message it seems that you have multiple accounts with code 465 so this is the cause of the error. If so you should change the code of some of them to avoid the duplication and the import will work.
If you are trying to migrate data from an existing system maybe it’s worth to use a proteus script. The openerp2tryton one developed by B2CK is a good example