Overdubbing problems

I record track 1 using the computer microphone input as the source.

Then arm and record on track 2 while listening to track 1 on headphones. The source for track 2 is the computer microphone.

Two problems:

1. When recording track 2, the first few seconds of track 1 are silent then suddenly track one can be heard. It is as if the first part of the track is muted although it all plays fine when not also recording on track 2.

2. Track two ends up with both what I recorded to track 2 AND track 1 (at somewhat reduced volume). I have verified that the microphone is not picking up sound leaking from the headphones when I am recording track 2.

How can I record track 2 while listening to track 1 without track 1 being recorded onto track 2?