Serial.write displaying repeating output from Xbee communication
Code is correct and it does what it is asked to. The receiver code reads the RX pin of arduino which is connected to the Xbee. The same pin is used by the FT232 interface IC to send data to your computer through USB.

The "hello world" you send is received by the Xbee and is send to microcontroller and FT232 at same time as that pin is shared between them. Thus the "hello world" will be seen in serial monitor, first time from Xbee and second time, from Serial.write(Serial.read()) of the program.

Also check whether the settings are correct from this tutorial, as Xbee will not be configured as P2P.