Group text conversation is adding myself as a duplicate participant

This just started happening tonight.

I have a regular group message with some contacts. Let's call that conversation A.

A new thread just appeared tonight, let's call that conversation B.

A's members are: Me, Person 2, Person 3, and Person 4 (4 participants including me)

B's members are: Me, Person 2, Person 3, Person 4, and my phone number (5 participants, 2 of which are me)

B just appeared out of nowhere. Since it includes a duplicate "me," whenever I send a message, I automatically get another copy of it back.

My group does not see the duplicate "me," only I do. This is what is most frustrating, it only created a new conversation with a duplicate "me" for my phone.

Some notes to keep in mind:

• We all have iPhones.
• These are NOT iMessages, these are regular text messages since one of us has iMessage turned off.
• When I send a message in conversation A (the original), the others see it, but their replies go to the new conversation B (the new one with duplicate "me") on MY phone only, not anyone else's. Everyone else seems to have the same, original conversation.
• If I try deleting conversation B and send messages in the original A, conversation B reappears when someone else replies.
• I cannot remove myself or anyone else from this group conversation, even though it has more than 3 total participants. (Does this only work for iMessage conversations?)
• I checked the details of each member of the conversation, and all the receiving info is set to their phone numbers. It has to be, since this is a regular MMS/group text conversation, not iMessage.
• I checked my own phone settings, I am only set to receive iMessages at my phone number. But, again, this should not be an issue since this is a regular text message conversation.

Has anyone else had this problem before? I don't know if it is a carrier issue since iMessage is turned off on one of our phones, but why would only my phone add a new conversation with another "me" as a contact, for a regular texting conversation?