How are the spectra of different elements discriminated in starlight?

Hello naked astronomers!
 
my question is regarding spectroscopy, since the light coming to earth from the stars is coming in "one go", how do we separate the different materials composing that star? lets say that the absorption light spectrum of oxygen and carbon put 'one on top of the other' makes the same pattern as helium, how then do you know if your light spectrum from that star shows that the star has helium or you are seeing oxygen and carbon through the spectrograph, since again the light comes in in "one go" towards us.
 
thanks very much, keep up the great work,
Levy (Israel)