Here’s what I mean:
Rachel is hitting/yelling/insulting etc.
So you say:
“I don’t know what happened to my Rachel. My Rachel is this nice sweet girl. She talks nicely, she shares with her siblings, she picks up her toys… And do you know what happened now? I lost her and instead this other girl came. She looks like my Rachel and she has the same clothes as her and her voice is the same. But my Rachel doesn’t act like this. So you know what? I want you to leave. Go! Leave! (And send her out of the room.)
And then either ask her to please send your Rachel, the “nice” Rachel back or call out for the “nice” Rachel.
And when she reappears, give her a huge hug and tell her in detail and with lots of exaggeration what happened.
“Oh Rachel, I’m so happy to see you!!!!!
Do you know what happened???
There was a girl here. She looked just like you! She was even wearing the same clothes as you! And I thought maybe it was you! But it wasn’t you! Do you know why? Because she was hitting and she wasn’t talking nicely at all. And I know that my Rachel doesn’t do that. So you know what I did? I sent her away. I told her to leave and to never come back. And oh, am I glad to see you…”
After doing this once with one of my kids, the next time she acted up she suddenly stopped and on her own asked me to call the “nice” Rachel to come!
Try it in your home and let me know how it goes…