Adobe Premiere Fill Left/Right Effects
The audio fill effects can be found in the effects window, under Audio Effects > Stereo > Fill Left / Fill Right. To apply either of these effects, select the appropriate clip in the timeline and drag the effect onto the clip (or drag the effect into the Effect Controls window).
The Fill Left effect takes the audio from the left channel and duplicates it on the right channel, deleting any previous audio which was on the right channel. The Fill Right effect does the reverse, applying the right channel audio to the left channel.
The most obvious use for these effects is when you only have audio recorded on one channel. For example, if you have an external microphone plugged into the left channel of the camera (and assuming the camera only records it on the left channel), you will probably want to duplicate it on the right channel as well in Premiere.