Trigger creates duplicate records

Question

Mysql "update" Trigger creates 8 records each update. The first record is the old one, the next seven are copies of the change. As I leave only the old and the edited?

I tried to put UNIQUE key on datetime from the "historical" table, but it generates an error. But neither is this solution what I need. I cannot name a Unique key because the same record can be changed several times.

Code entered in Phpmyadmin:

DROP TRIGGER IF EXISTS `placasptc`.`update_bds`//
CREATE TRIGGER `placasptc`.`update_bds` BEFORE UPDATE ON `placasptc`.`bds`
 FOR EACH ROW begin
insert into historico
set hist_action ='update',
bkp_ID = old.ID,
bkp_empresa = old.empresa,
bkp_prefixo = old.prefixo,
bkp_placa = old.placa,
hist_modificado = now(),
hist_usuario = Current_user();
end