You're welcome, but to be clear I never recommend anything I do not do or would not use myself. However, since I asked for trouble by suggesting that app I purchased it and have been testing it the same way you described — on a sacrificial system.
It appears to do what it says, and I haven't encountered any problems with it. It finds apparently duplicated files and identifies them by name, location, size, and modification time stamps, after which you can decide what to do with them. Still, I would be very cautious about deleting things that have the slightest potential to destabilize a system or affect any apps the charity may require. For example, deleting images in the Photos Library has the potential to corrupt its database, although I wasn't able to draw any conclusions with it during my limited testing.
Given your task, which seems to involve multiple Macs having been used by multiple people, likely with little or no regard to file organization or housekeeping, it is likely to be a useful tool. Just be careful with it, and if in doubt about anything I'd err on the side of caution. I wouldn't let it do anything "automatically" (assuming it offers that capability).
Time Machine or the equivalent remains an essential prerequisite.
