Several solutions see to do the job.
I created a 2.7 GB directory of various duplicated files and ran them against each other.
Fastest: Singular, closely followed by dupeGuru. MrClean and FileBuddy trailed a long way behind.
Easiest to use: Singular starts off with a window asking you to drag your files into it. Super-simple. dupeGuru has more options on the initial screen which can be cumbersome. For example, you have to select which directories you wish to scan. However, the additional options allow you to add two different directories and also sort by size – very useful. MrClean and FileBuddy again have a rather complex initial setup.
Cons: FileBuddy obviously suffers from being an all-in-one application.
In conclusion, for a single directory, Singular is by far the easiest. However, if you wish to sort by size and/or check for duplicates on multiple directories then with dupeGuru wins out. One other consideration is that Singular has occasionally crashed with big directories whereas dupeGuru completed a 62GB directory scan comfortably.