How do you record beginning from where you left off?

When I press record again it seems to record on a different track from the beginning, and then when I play back it plays both recordings at the same time.

2 ways:

  1. Press the Pause button (or “P” key) rather than the Stop button if you just want to pause the recording and then resume.
  2. If you need to “Stop” then resume, use Shift + R to “append record”.(also available in the Transport menu).

Thank you. How do the same on Mac?

P, Stop, and Shift-R work on all three computers.

I did that on Mac and it suddenly played back a second of what I just recorded as I was recording.

I think you are describing a known Audacity bug. Try turning Overdub off (in the Transport Menu). Does that work for you?