shipment plugin development - same confirmation email triggered many times

[Aelric]

Hello there,

Works on a new shipment plugin. I am facing a strange behavior: the same confirmation email is triggered many times. If someone knows how are triggered these emails that would help me to find my bug.
There is only one command recorded in my xxx_virtuemart_shipment_plg_xxx table but one reference to each of these mails in xxx_virtuemart_order_histories table.

Thanks in advance,
Aelric

Configuration :
Joomla 3.4.1
Virtuemart 3.0.9
PHP 5.4.27