Hey there! Usually in chiptune trackers this is done one of two ways:
1. The easiest way is to copy the phrase/melody into another channel, moving it a few rows down, and lowering its volume, this produces an echo of the original, and it gives you a lot of control over how it sounds. For example you can pan the echo to make the phrase sound wide and spacious, or detune slightly to give it a shimmering effect.
2. This only really works for arpeggios or other rhythmically even patterns; but if you have a pattern with gaps in between each of the notes, like (C _ E _ G _ B _ C _ E _ G _ B) you can repeat the notes 'inside the gaps' at a lower volume, like (C _ E C G E B G C B E C G E B G _ B). This essentially does the same thing as 1, but it can fit all inside one channel.
This video by Ben Burnes explains it in Famitracker, but the general idea will be the same in WaveTracker and many other chiptune tracking software.