I am getting the message "More than one device is trying to play music." in the Music app on my Mac once or twice a day. I am playing from the Music app on my iMac and also have the output directed to my stereo HomePod setup in another room. All the devices are registered to the same AppleID (mine) and there is no other AppleID involved. So the Family Sharing suggestion that appears is irrelevant. For what it's worth, I most often see the message just before midnight (PST).