Endnote doesn’t duplicate a record in two groups, but no, there is still (despite numerous requests) no easy way to combine two records retaining unique info, including group association and removing the copy. The best I have done, is to edit each group to have a unique keyword to associate it with the group then find duplicates, and update the copy with the PDF (including the addition of the missing group keyword) and remove the duplicate. They use smart groups to find the keyword to create a group. I then “freeze” the members of the smart group in a regular group, by dragging and dropping from the smart group to the “frozen” regular group. you can do this by copying all and dragging to the frozen version, and it will only copy new ones to the group. It won’t duplicate them.
When I import new citations, I similarly globally edit the keywords so they automatically get incorporated into the smart groups and move them to the frozen group.
automation of these steps in Endnote would be welcome!