@crinrict, Update on what I'm seeing in my game. Got flea market notice. Grouped family and sent them to Planet Honey Pop and could pick from group to sing duet with. Sent group to Waterside Warble and only option to pick for a duet partner was the bartender, nobody from group. Sent them to flea market, even though no active sim can sell anything bug present. As end neared I had the group shoot baskets. After everything flea market related disappeared I sent the group to the venue and was able to then pick another group member to sing a duet with.
Geek Con came to town. Sent a group to Waterside Warble and could pick group member to sing duet with. Sent to Planet Honey Pop and only option for duet was the bartender. After Geek Con was over, I had group go back to Planet Honey Pop but unlike what happened with the flea market, nobody from the group appeared as a choice to sing a duet with. Disbanding the group and then reforming didn't allow for a group member to be shown for picking to sing a duet with either.
The problem does seem to happen when a sim group attends a festival and then goes to the karaoke venue near whatever festival it happens to be, or, goes directly to a karaoke venue when a festival near it is in progress. Why waiting until the festival is over to go sing karaoke doesn't allow a grouped sim to be chosen for a duet, as with what happened after the flea market items cleared, I don't know.
Unless you have a specific scenario you would like me to try, I won't be testing further. I now know if I want a grouped sim to sing a duet with another sim in the group, all I need do is avoid karaoke venues where a festival is nearby.