GSM SIM-300 read msg problem
See http://arduino.cc/en/Tutorial/SoftwareSerialExample

By using the same serial port to communicate with a modem and to communicate with your PC, you  are causing confusion.


You are echoing the modem's reply back to the modem.
Modems usually echo commands back to you.


I'd use separate channels as shown in the Arduino serial tutorial.

Last time I did something like this (with an Arduino communicating with a MIDI serial device) I used a 16x2 LCD to display what was being received. But you could use software serial if you prefer (or don't have a character LCD)