Have you experienced this? I've had it with my MIL and now with a friend of mine.
It usually happens when I'm warning the DC to behave so..."Girls...please stop jumping on that sofa immediately before someone's hurt or something gets broken"
I'll get half way through the warning when my friend begins SPEAKING OVER ME but saying basically the SAME FRIGGING THING!!
"Girls you need to stop doing that...."
Wtf?
She did it yesterday and it just came back to me....I used to say nothing but I had to speak up so I stopped addressing my DC and turned to my friend and said "You don't need to join in when I'm parenting, I have it under control...now I'll start again...." and I told my DC what I was telling them without interruption.
WHAT IS THIS!? Why do people do it? Well....MIL and friend...separately. I had to tell mil too and she's stopped now.