Make sure cp isn't aliased on one or the other machine - run this command on both:
Probably on one machine has cp aliased to "cp -i". You need to find where the alias is set and comment out / delete that alias. Typical places where aliases are set:
- ~/.bash_aliases ~/.aliases or something similar.
Assuming you're using bash as your shell the first three will be checked when you start a log in shell. Often aliases are split out into a separate file which is called from one of the first three files, hence the name of an aliases file if present is not certain - it depends what the author of the main file decided would be nice.
A quick way to try to find the alias:
cd ; grep 'alias cp' .bash* .profile .*alias*