Review duplicates
The "Review duplicates" utility lists assets that Immich has found to be visually similar, and lets you choose what to keep. Open it here.
For how detection works, how Immich decides what to preselect, and what happens to the metadata of the assets you trash, see Duplicate detection.
Reviewing duplicates
The review duplicates page allows the user to individually select which assets should be kept and which ones should be trashed. When more than one asset is kept, there is an option to automatically put the kept assets into a stack.