Is iMyMac legitimate and does it work?

macOS does not need the help from any third party junkware like iMyMac (aka) MacCleaner, MacKeeper, etc.....

In fact, we've seen and helped countless folks around here, that have had nothing but problems using those so called third party clean-ware and windows based AV-ware.

No it’s worthless as are ALL third party antivirus, cleaning, optimizing and other “ maintenance” apps. The best tool for keeping Mac OS in good shape is Mac OS itself. Simply keep Mac OS up to date and otherwise please leave it alone.

You're welcome.

Always make sure that you have good backups in place, before you decide to experiment with any kind of third party app's that promises to delete, remove or reorganize anything.

You can take a look at the Mac App Store, but beware that even some of those App's might not give you good results.

Where are you seeing duplicates in Photos?

How many and do you have any idea how they got there?

FWIW, personally I think that manually removing or living with some duplicates, would be better than using something that could bork the whole library structure..

What app are you using to manage your photos. We have no idea where you store and manage your photos so we rely on you to help us understand so we don't have to write 20 questions. :-)

As a suggestion, sometimes it helps to review Writing an effective Apple Support Communities ... Apple Support Communities so you get an idea what we need from you when you ask a question on these forums.

You are not talking to Apple on these forums, we are all end users like you. I’d recommend calling AppleCare and speak to someone from Apple.