hi my question is about duplicate photos on my mac

The Duplicates album does not show up in the Photos library sidebar on your iMac until the library finds duplicates. Depending on how long you've been using the library an how big it is it could take some time before it appears. The library also is searching for objects and people.

Also if you're using iCloud Photos it could take even longer.

If you want to more quickly check for duplicates you can use one of the two apps below:

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 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.

It’s not on the side menu bar. But rather you have to double finger click on the photo you want to duplicate. Hope this helps!

You're welcome.