Text messages add unintentional recipient

[AC Question]

This is not the issue I've seen others report where the text app randomly selects the recipient and sends the text to the wrong person. If I start a new text, key in the recipient's name, write my text and click "Send," the "To" field will sometimes show another recipient has been added. That is, the text will go to the originally intended recipient, but another person will get it, too. I can't delete the unintentional person from "To," because the new name is not visible until after the text has gone out. The messaging app will add someone who has been, in the past, part of a group text with the intended recipient. After the text goes out, the text thread will show the conversation history with those two people. That is, I can see the previous conversation I've had with them, and the new text is now part of that thread. This is happening randomly, about 50% of the time. There does not seem to be any pattern to when this will happen. Again, the unintentionally added person will not be a random person from my contact list...the phone only adds people who I have previously group texted with in the past.

Bill S.
LG-D850
Android version 6.00