You probably recorded a mono signal on a stereo track. A STEREO track, by definition, contains two channels, a left and a right. Recording with one microphone, or one guitar, gets you a MONO signal. In other words, you get one channel for each signal source.
If you are using a single source, you need to set up Sonar to record a mono track. To do this, you first need to understand how the I/O assignments work in Sonar. Please see the explanation in article 31499. Make sure that for a track’s input, you choose a mono input channel (mono inputs have “left” or “right” in their names, not “stereo”). If this does not help, you can also make sure the Interleave button is set to Mono on your audio track. The interleave button can be found right next to the phase button on an audio track.