SMS texts to my number add someone else

Recently I've noticed that when I recieve some sms texts to my number, there is a third party also being added unintentionally. It's at an address RCP[MyPhoneNumber], an address I have absolutely 0 connection to. This is, understandably, a worrying breach of privacy. 

I use facebook messenger for my sms texts. I've looked through the account settings, and haven't seen anything that would link my account to another's. I'm wondering if it's something on the network's end that is directing this user to recieve sms messages addressed to my number. 

Re: SMS texts to my number add someone else
Customer Service Rep

Hello, BlueVirtue. We can certainly understand wanting to ensure your privacy is protected and are happy to assist. Does this only occur with incoming messages? Have you tried any application aside from Facebook Messenger for texting? StanleyC_VZW

Re: SMS texts to my number add someone else
Champion - Level 3

Looking up the domain, its registrar is www dot joker dot com.  You can report this with email to abuse at joker dot com.

...Just another VZW customer...trying to offer some assistance...