I prepared and attached a script file that enables debug logging while the smscon daemon processes sms comands. To use this debug script you need the ability to become root. Usage:

1) copy the debug script somewhere onto the N900 device and name it "smscon1.sh".
2) start xterm
3) become root
4) change directory to the folder containing the debug script.
5) enter command: 6) enter command: 7) follow the instructions given by debug script.
8) go to 6) in case you want to repeat debugging.
9) post the generated smscon-debug.log file here that I can analyse it (it is anonymized).

I hope that this will help us to find out reason of twice sent SMS.