So the idea is that someone posts a band and says Marco! In the subject line. The follow up is a sufficiently similar band to which the response is (obviously) Polo!
For example:
Marco!
The Who
Polo!
Greta Van Fleet.
Marco!
Johnny Cash
Polo!
Colter Wall.
Feel free to start. 
I’m not sure i really understand why, what is the point?
Fun would be the point, an amusement to consider similar bands and maybe, out of curiosity, find something new to enjoy.
I suspect it works better when (a lot of) alcohol is involved…
Perhaps! Chris is right; it just struck me as interesting one morning. And I wondered how many artists have a doppelgänger out there. I can think of a few more, but I was interested in polling the group mind to see what it stirred up.
Perhaps it will fall flat. We’ll see. I’ll start.
Marco!
Dire Straits.