Is It Possible For Two People To Have The Same Number?

I want to know if it is possible for two people to have the same number with each other? I ask this question because i would call my friend and we would would talk everyday. She would tell me how sometimes when people call her number a different person would answer. That never happened to me until today. I was also wondering this because the different person got a hold of my number even though i do not know him at all.

He called me awhile ago not today but me and my friend got freaked out. I am advising her to change her number but she doesn't have enough money to change her number. Is there a way AT&T would change her number for something like this?