I haven't been active for a long time now so this may have changed.
Back then the reason behind locking rather than deleting was because the duplicate may be asked in a different way, using different keywords. So keeping it viewable allowed people searching for an answer to effectively have multiple entry points into finding the answer because perhaps they were googling the wrong combination of keywords to get the original to show up, but the duplicate would match.
Back then the reason behind locking rather than deleting was because the duplicate may be asked in a different way, using different keywords. So keeping it viewable allowed people searching for an answer to effectively have multiple entry points into finding the answer because perhaps they were googling the wrong combination of keywords to get the original to show up, but the duplicate would match.