Minicards and SL80xx modems are customized to attempt to send SMS multiple times. In case when SMS is not sent immediately, modem will attempt to send SMS again after predefined wait interval has elapsed. AT!SMSRETRY command defines such modem behavior.
If you want to customize modem to try to send SMS only once, you can define such behavior with AT!SMSRETRY=0,0
Please refer to AT command guide for detailed AT!SMSRETRY description.