My wife and I both use Apple phones. One thing I don't understand is why a friend can receive the same text message from his lover's mobile phone.

Hello! According to your situation, your two mobile phones use the same account. You can turn off imessage in Settings-Info, and this will not happen. Of course, in this case, you can't use Apple's imessage service, and you have to pay the mobile operator's SMS and picture fees like other Apple mobile phone users. Although they are husband and wife, it is recommended to use different apple id to avoid unnecessary misunderstanding and affect feelings.

I hope my answer can help you and be adopted!