The “proper” way to double an instrument like a guitar or a vocal is to record it once, then go back and try and match it exactly with a new track. The soundwave should be almost the exact same as the first but just slightly different.
This slight difference will allow it to be percieved as a bigger and wider sound to the ear.
After you double it, pan one of the sounds all the way to the left and the other one all the way to the right. You can also vary the panning a bit depending on how you want it to sound. A lot of experimentation is good here.
If you can’t go back and record a second take for the double then there’s a workaround by doubling the exact waveform.
To do this, make a copy of the instrument and move the waveform forward or back by a few milliseconds. Watch out for phasing issues because it is the exact same waveform.
Some people think this method is tried and true and very legitimate so try it for yourself and see how it sounds.
A lot of people do this with vocals as well as guitars for big, huge wide sounds. If your mix is thin or lacking life, try this. This is a big Nashville trick..