sharing a email domain with someone else without knowing

I've noticed that my iCloud email is also affiliated with someone else's account. I know my email should have the domain of @me.com and @iCloud.com. I've always used the @me.com domain when formatting my emails but I noticed that another individual uses the same address as my email (kclaiborne) but just uses a different domain. at 1st I didn't think anything of it until I received an email about a iPhone being shipped to a whole other state. I thought it was spam but upon further research the email sent to me was legit and gave me red flags. I reached out to customer support about this and was told to not be alarmed but my concern was if I'm receiving their emails (receipts, confirmations, etc) of important information, then who's to say they're not receiving mine on their end. I just want to know is it possible to share the same email with someone else and the apple system not know it?