I plugged in headphones to the jack and sound came out of both the headphones and the internal speakers. how do I fix this? I've tried multiple headphones. this didn't happen before
Sounds like your headphone jack might have something in it, this happens a lot. Check to see if there is anything in the port wipe it out and try again.
If that doesn't work, check your sound settings, make sure you don't have 2 different outputs going out instead of one.