How do i remove i386 packages from my Pi?

Question

I have a Pi 3 Model B running Raspbian Stretch. When I run sudo apt-get update, I see my Pi downloading i386 packages along with the armhf packages, and when it's done fetching all the packages it tells me that it skipped 'rpi/binary-1386/packages' because 'InRelease does not support i386 architecture'.

I'm not entirely sure how i386 packages got added to the list. How can I remove it so the Pi only gets needed packages?

EDITED This is the result of grep -e "deb " -e "deb-src" -r /etc/apt

/etc/apt/sources.list.save:deb http://raspbian.raspberrypi.org/raspbian/ stretch main contrib non-free rpi
/etc/apt/sources.list.save:deb-src http://raspbian.raspberrypi.org/raspbian/ stretch main contrib non-free rpi
/etc/apt/sources.list.d/raspi.list:deb http://archive.raspberrypi.org/debian/ stretch main ui staging
/etc/apt/sources.list.d/raspi.list:#deb-src http://archive.raspberrypi.org/debian/ stretch main ui
/etc/apt/sources.list:deb http://raspbian.raspberrypi.org/raspbian/ stretch main contrib non-free rpi
/etc/apt/sources.list:#deb-src http://raspbian.raspberrypi.org/raspbian/ stretch main contrib non-free rpi

Answer 1

This usually happens when you (or a script run on your behalf) add a foreign architecture to your package manager. You can check it out with

dpkg --print-foreign-architectures

If you don't need i386 anymore, you can remove it with

dpkg --remove-architecture i386

Be sure to remove all i386 packages before you remove the architecture.

Further reading: Multiarch/HOWTO