Dear Apple!

You're not writing Apple as this is a user to user only forum.

Per Apple however, it was noted that if a twin is identical enough, it would be possible that both could access iPhone X by Face ID. If this is a concern to you, you might want to use a passcode as opposed to Face ID. It is not likely your phone is defective.

Face ID is not a perfect system and Apple has acknowledged that. THey're still refining the process but it's a little difficult with twins as well as close relatives (people just look similar). So it's not really "expected" but there's not really a lot Apple can do.

I've seen that reported before. But you're not talking to Apple here, just other users like yourself. You can let Apple know here:

Product Feedback - Apple