Phone getting unlocked with 2 Face IDs

This support article discusses the issue you are encountering --> About Face ID advanced technology - Apple Support

"The probability that a random person in the population could look at your iPhone X and unlock it using Face ID is approximately 1 in 1,000,000 (versus 1 in 50,000 for Touch ID). As an additional protection, Face ID allows only five unsuccessful match attempts before a passcode is required. The statistical probability is different for twins and siblings that look like you and among children under the age of 13, because their distinct facial features may not have fully developed. If you're concerned about this, we recommend using a passcode to authenticate."

If you have a family member, especially a child, they may be able to unlock the device given either similarity to you or because the device is unlocked by the other user using a passcode and is combining the two faces. Recommend setting up Face ID again.