Four stages
Files of a unique size cannot have a twin and are ruled out first. What survives is read at both ends, then hashed in full, then — below a size you set — compared byte for byte. The expensive work only happens where it can still change the answer.
Clones
APFS can hold two names for one set of blocks. Removing one frees nothing, so DiskDuo marks clones and leaves them out of what it offers to reclaim.
Which copy is kept
Choose the newest, the oldest or the shortest path. The rest are staged for review; nothing is removed as a side effect of the search.