Duplicate photos created during transfer from iphone

Do the duplicate pair have the same file name?

As far as the Duplicates folder in the sidebar it often takes a while before Photos finishes its search for faces, objects and duplicates. Leave Photos open and in the background while you're using you MBA and leave the MBA on at night (do not sleep) with Photos open in the background.

There are a couple of good 3rd party apps for identifying duplicate images.

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.

Repeating what Old Toad said, it may take awhile for Mac's Photos to identify duplicates. If there is not Duplicates view in the Sidebar, then it hasn't found any.

Usually Photos will not import duplicates if you go through a normal import process. Mac Photos does a pretty good job of finding duplicates, so you may want to wait a bit, perhaps a couple of weeks or more.

In the future, try using the app Image Capture that's in the Applications folder rather than the Finder sync process, and see if you have more control over what's being detected and transferred.