Duplicate files · Removing and restoring duplicates
How do I remove the extra copies in one duplicate set?
Short answer: Select the copy to keep with the round selector, click the set's Move … to Recycle Vault button, and confirm. The other copies go to the Recycle Vault, recoverable for about 6 months, and the set leaves the list.
In the set, make sure the copy you want to keep is selected with the round selector (the recommended copy is selected for you), then click Move … to Recycle Vault at the bottom of the set. A confirmation names the copy being kept and how many others will go; confirm with Move to Recycle Vault.
The set disappears from the list and the space it was taking is freed. Everything removed stays recoverable from the Recycle Vault for about 6 months.