How do I overdub an existing track with itself?

I successfully did this a few weeks ago on Audacity, but I’ve forgotten how I did it. I’ve tried unsuccessfully over the past hour or so, but I seem to have forgotten. I do remember I had to align the initial track and the second recording of the initial track (that was easy to do). I remember pressing “record”, after doing one or two other things, and the second track was showing on the screen that it was recording the initial recorded track.
I’m not referring to “duplicating the track” (there’s an Audacity duplicating function, but that doesn’t make a solo performance turn into a “duo” sounding performance like overdubbing does). The sound I achieved with the overdubbing was a “doubling” of the sound … 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.
So, can anyone tell me how to overdub an already recorded vocal/instrumental track with “itself” (I’m also not referring to recording another track with me singing and playing again … I’m referring only to the initial track being overdubbed with itself in order to produce a doubling of the sound … like solo to duo.

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 ).

Thanks for the reply, however a chorus effect is quite different from what I’m referring to. I’m referring to an acoustically 100% accurate re-recording of the initial recording (by “recording” I’m referring to me singing and playing acoustic guitar). That can of course be achieved by me singing and playing in time with the initial recording (a simple overdub) … it ends up sounding like 2 of me and acoustically very accurate and natural sounding.
But I was experimenting a few weeks ago , and found a way to re-record the initial recording itself, and then all I had to do was match up the timing of the initial recording and the second recording. The result was 100% acoustically accurate, perfectly acoustic sounding and it saves me the hassle of having to sing and play in time with the initial recording in order to get a '2 people playing" result. When I tried to do it again today, I couldn’t. I do recall that it wasn’t hard to do, not complex at all, but the exact procedure in Audacity seems to have escaped my memory. If it comes back to me I’ll write it down so I don’t forget.

If you turn the speed to zero you lose the chorusy effect,
and just get delayed “100% acoustically accurate” versions left & right …

suggested settings.png
NB: you must duplicate a mono track, & join them to make a stereo pair, when using any stereo-effect.

Problem solved. I’ve just remembered how to do it. It’s very easy. Now if I record an initial solo singing track, then record that track over itself, it instantly ends up sounding like 2 people singing. Duplicating or copying the initial recording doesn’t produce that same sound of 2 people singing.

You’re effectively adding a delay equal to the latency of your computer.
That delay time is fixed, whereas if you used a plugin, or time shift tool, you could adjust the delay between the original & the duplicate to your own taste.

Another method to add delay is to generate silence at the start of one of one track, (a small fraction of a second. say 20ms).