How can I delete duplicates

It depends on what system you're running? If you running Ventura Photos V 8.0 has a duplicate finding capability and will merge duplicates into one file while keeping all metadata and album participation intact. It can take some time for Photos to complete it scan and make the Duplicates folder visible. So keep Photos open and in the background at night (don't sleep the Mac but just put the screen to sleep.

If you're running an older system than Ventura I've run tests on the these two apps with the following results and found them to be safe to use:

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, both original and edited versions, along with their metadata between libraries.

PhotoSweeper - $9.99 - Demo version available.

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.

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.  The two apps above fit the bill.

I've been using DuplicateFilesDeleter for a few months now and it's been a lifesaver! It's so easy to use and has helped me free up so much space on my computer by deleting duplicate files. I highly recommend it to anyone who's struggling with cluttered files.

Be very very careful with this! I haven't used it, but I know that most apps that "clean files" do not take proper care of the Photos Library. Deleting or moving files in the Photos Library package may permanently corrupt the Library and make recovery very difficult or impossible! I notice that their web page doesn't address this at all.

Old Toad suggested the apps he did because they have proven safe for Photos.

The only DuplicateFilesDeleter I've found online was for PCs.

I don't know what you're referring to but the only two I can recommend are the two I've already posted about. They have been tested and found to be save for Photos libraries.

And for general cleaning Richard.Taylor is right on in that many of them remove duplicate files indiscriminately without considering if they are needed or not. I categorically cannot recommend a general cleaning app. Rebooting frequently cleans out temporary system and application cache and swap files and that's sufficient. A Safe Mode boot also does some repairing as well as cleaning.