I know it's feedback but is there anyway to stop it?

It happens a lot with my semi-acoustic and somethimes with my SG.

I try to play with another lad (him on electric-me on acoustic)
and because of the feedback we just have to stop.

Any advice?
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Turn down the gain, stand further away from the amp when you play.

edit: stay on the bridge pickup
Fender Deluxe Players Stratocaster
Epiphone G-400
Laney VC30
Boss DS-2
put a buffer between you're guitar and the amp. i dunno what you can use as a buffer though, or even if that's the right word. I think it's baffles. just something that stops so much sound going back and forth between the amp and guitar