hearing other tracks when recordin multitrack

I am recording in multitrack: I sing and record the first track, then I record a second track while I listen to the first in the headphones and so on.
However, when I playback the second track turning off the first track, I can hear the first track in background, very weak but it’s there. Why? Does it depend on the mixer setting? I am using alsamixer, and I did not find any setting that seems appropriate.
I also had the doubt that it could be a playback problem, so I deleted the first track, but the sound is still there, very weak but audible in the second track.
Thanks for your help

Perhaps the first track playing through your headphones while you record the second track is being picked up by your microphone.

Hi, do you mean picked up acoustically or through electric interference?
I think the first is very unlikely: I wrote headphones but I meant earphones. To pick up sound from the earphones inserted in my ears the mic should have a huge sensitivity. If you mean coming from an interference that is indeed more credible.
I might try to repeat the recording/playback with the earphones disconnected to determine if the earphones cable is acting as an antenna, or if it is a software/mixer problem. Does that make sense?

Good idea.
Also test it with the Track Gain slider (http://manual.audacityteam.org/o/man/audio_tracks.html#gain) turned down to minimum on the first track. This will test if there is some “bleed through” inside the computer (which could be software or hardware).