How do I overdub an existing track with itself?

[Trebor]

… in other words the solo vocal sounded “solo” on the original track, but sounded like 2 people singing and playing when I overdubbed the initial track with itself. I’ve just tried “duplicating” the track but that does not achieve the same sound…

I think chorus effect is what you’re looking for.

( NB: It is necessary to duplicate a mono track into dual-mono before applying a chorus effect ).