I have an issue that has been puzzling me a little. My mother has a Samsung Galaxy J3 and my father liked it, so he bought one as well. They both use their own SIM cards, in their respective phones and don't share any accounts other than a Facebook page. However, my dad suddenly ended up with all my mom's contacts and phone numbers. And when he deleted them they disappeared from her phone as well. On the other hand, all his contacts didn't get on my mother's phone. So it seems to be a one sided thing.. I can't figure out for the life of me how the two phones got synchronised and can't think of how to fix it. Can someone maybe help?
Are they using the same Google accounts @Arthame? Because on setup of the phone he might have used your Mum's Google account and that be the cause of them being synched.
Further to the suggestion by @John4Oh, please go to Settings > Accounts and check to see whether the 2 devices have a shared Google or Samsung account. If this is the case then you can remove the account from your dad's phone to prevent the contacts from syncing to his device, or go to Accounts > Google > Select the email address > Untick 'Contacts'.