Group text streams merging unexpectedly on recipients

I have two separate text streams: one with five people and one with a sixth person included. Today I sent a new text stream to the five people and a few minutes later the sixth person was added by the system somehow. It also merged yesterday ‘s text stream which included the sixth person with today’s text stream on the other recipients phones, so now the two messages are combined and all six are included on the other phones, but still separate on mine. Very frustrating!