To safely identify duplicates you want an app that will identify the potential duplicates, put them in an album or mark them with a keyword for easy retrieval and deletion by you. You don't want one that does the deletion itself for obvious reasons.
I've run tests on the following apps with these results:
PhotoSweeper - $9.99 - Demo version available.
PowerPhotos - $29.95
PowerPhotos is the iPhoto Library Manager version for Photos and is very powerful. Although more expensive I would recommend it as it has more capabilities than the others like the capability to merge Photos libraries or copy photos along with their metadata between libraries.
PhotoSweeper compares bitmaps and/or histograms so it can detect duplicate images even if they have different file sizes, file names, image sizes and capture dates.
Both of these are safe to use on a library as with each it's the user's final say in deleting the files or not.
Also as long as one has backups of their drive, i.e. with Time Machine or other backup app, a damaged library can be restored from the backup made just prior to the damage occurring.
