How Do I Use 'cp' to Overwrite Files?
I'm stumped as to why cp is behaving this way if there's no alias involved. However, there is a quick-dirty-and-dangerous utility provided for this exact situation: yes. You can use it to pipe a continuous stream of affirmatives into any command that gives you confirmation prompts (you can use it to send any text, but the default is "y"). 

yes | cp foo bar will execute the cp command and bypass all confirmation prompts.