Using GarageSale Version 7.0.14b3 (831), macOS Version 10.12.6 (16G1212)
In the Image popup menu, Can you limit the “Open in External Editor” applications to those on the Startup Disk (Main Internal Boot SSD drive), if there are duplicate applications? I have a “backup drive” always mounted and this menu is showing all applications on both drives, hence they are all duplicated. But I have no way of knowing which application is on my Startup Disk (Main Internal Boot SSD drive)… Also, can the list be alphabetized?
However, there is no easy fix for removing applications from your backup drive. We just ask the system for a listing of application that can edit images, and display that list.
Maybe un-mounting your backup drive while not in use would be a workaround.