It really depends on what the end goal is. Making a mono sound have width is tricky(at best) and some things might not translate well if the final mix gets played back on a mono system(the track might simply sound mono, or it might disappear due to phase cancellation). Duplicating by itself does nothing. If you take the duplicate and nudge it in time(600-1600 samples) will help some. Using Elastic Pitch to change the pitch slightly(6-11 cents sharp or flat) will help some. If you are the player, then double-track the part. If its a guitar track, play it again. If its an electric guitar part, change amp settings or use a different guitar or pickup setting. If its a vocal part, double-tracking is the ideal way. There are some plugins that create a double so you might demo some of those and print the results(Izoptope Vocal Doubler, ReVoice Pro and Sonnox Voxdoubler are 3 options).
Mono Electric guitar tracks will work great without doubling if you pan to one side and send to a mono reverb on the opposite side(Dverb works nicely). remember to use a MONO AUX track for this
