when you connect a headset to the headphone jack on the notebook, the sound should only go through the headphone. this is how a headphone jack normally works. but if the sound is going through both the headphone and the speakers, this to me sounds as a problem with the jack, which, i'm afraid could be hardware related.
i have seen many users having such a problem with the headphone jack. hence, i would recommend you to contact the notebook manufacturer and check if this is a known problem and the list of possible fixes.
however, one other thing what you could try is, using a different headphones. this will isolate whether the problem is with the pin or the jack.
