Use the built-in Duplicates album in Photos to merge copies and reclaim storage without losing quality.
Open the Photos app and scroll down to Utilities (on older iOS, tap the Albums tab and scroll to the Utilities group). This section only appears after the phone has finished scanning your library, which can take a day or two on a new device.
Open Duplicates to see exact and near-identical copies grouped side by side. If the album is missing, you simply have no detected duplicates yet.
Tap Merge next to a pair to combine it, or tap Select, then Select All, then Merge to clean the whole album in one go. Merging keeps the highest-quality version and combines the metadata from the copies.
Confirm the merge when prompted; the discarded copies move to the Recently Deleted album inside Utilities, where they sit for 30 days.
Open Recently Deleted, authenticate with Face ID, tap Select, then Delete All. Storage isn't actually reclaimed until this album is emptied.