I created a new vault called 'Duplicates', and used the Tools menu entry to tag duplicates. Then I selected all the entries with the "duplicate item" tag and moved them to the Duplicates vault.
There were more than 800 items, so it's taking a while. While I have faith it'll complete, not only is there no indication that 1Password is busy, but the UI is still responsive--I can change which entry is selected, etc. However, the actual display doesn't update. So switching from A to B shows B selected in the list pane, but still shows A in the detail pane.
While ideally a long-running operation would be entirely in the background, with maybe a progress pane added to the top or bottom of the display, I realize that's a lot of work to accomplish. At the least a modal drop-down with a progress indicator would be a big improvement because it would give me immediate indication that it's working, and I can't do other things until it completes.
Thanks for considering this!
1Password Version: 7.2 (latest)
Extension Version: Not Provided
OS Version: Mac OS X 10.14.3
Sync Type: 1Password Family Account & Dropbox