What is a good, trusted program to use to delete duplicate files on my Mac/iCloud?
There are no magic programs for that.
Best bet is to manually go thru your user files and decide what you want to delete.
If your Mac is low on free storage space, see > Free up storage space on Mac - Apple Support
TLDR; Fdupes && diff
Depending on your familiarity with macOS file systems and comfortability with advanced-user level tasks, you can utilize the Terminal.app and perform file/directory comparison commands to identify space saving opportunities.
Fdupes checks if the files are identical. Diff checks how files are different. Fdupes better for large number of files, use first. Then narrow down with the line by line differences in each file.
Here’s a well received resource.
Deleting duplicate files on Mac
